nat: fix memory leak and refactor nat44-ed db init/free.

nat: fix memory leak and refactor nat44-ed db init/free.

fix memory leak and refactor nat44-ed db init/free through.
how to reproduce memory leak: input "set nat44 session limit
50000 vrf 1" repeatedly.

Program received signal SIGABRT, Aborted.
0x00007ff4b3cc5337 in raise () from /lib64/libc.so.6
(gdb) bt
0  0x00007ff4b3cc5337 in raise () from /lib64/libc.so.6
1  0x00007ff4b3cc6a28 in abort () from /lib64/libc.so.6
2  0x00000000004079db in os_panic () at /usr/src/debug/vpp-23.02/src/vpp/vnet/main.c:417
3  0x00007ff4b43e784f in os_out_of_memory ()
   at /usr/src/debug/vpp-23.02/src/vppinfra/unix-misc.c:221
4  0x00007ff4b43a71aa in clib_mem_heap_alloc_inline (heap=0x0, size=27263040, align=64,
   os_out_of_memory_on_failure=1) at /usr/src/debug/vpp-23.02/src/vppinfra/mem_dlmalloc.c:613
5  0x00007ff4b43a7256 in clib_mem_alloc_aligned (size=27263040, align=64)
   at /usr/src/debug/vpp-23.02/src/vppinfra/mem_dlmalloc.c:635
6  0x00007ff4b522fafa in alloc_aligned_16_8 (h=0x7ff46a7815b8 <snat_main+408>, nbytes=27262976)
   at /usr/src/debug/vpp-23.02/src/vppinfra/bihash_template.c:59
7  0x00007ff4b522fd12 in clib_bihash_instantiate_16_8 (h=0x7ff46a7815b8 <snat_main+408>)
   at /usr/src/debug/vpp-23.02/src/vppinfra/bihash_template.c:163
8  0x00007ff4b5230037 in clib_bihash_init2_16_8 (a=0x7ff465f36870)
   at /usr/src/debug/vpp-23.02/src/vppinfra/bihash_template.c:245
9  0x00007ff4b52300ac in clib_bihash_init_16_8 (h=0x7ff46a7815b8 <snat_main+408>,
   name=0x7ff46a754871 "ed-flow-hash", nbuckets=262144, memory_size=0)
   at /usr/src/debug/vpp-23.02/src/vppinfra/bihash_template.c:260
10 0x00007ff46a7013e8 in reinit_ed_flow_hash ()
   at /usr/src/debug/vpp-23.02/src/plugins/nat/nat44-ed/nat44_ed.c:3264
11 0x00007ff46a7014fd in nat44_ed_sessions_clear ()
   at /usr/src/debug/vpp-23.02/src/plugins/nat/nat44-ed/nat44_ed.c:3299
12 0x00007ff46a701044 in nat44_update_session_limit (session_limit=70000, vrf_id=1)
   at /usr/src/debug/vpp-23.02/src/plugins/nat/nat44-ed/nat44_ed.c:3225
13 0x00007ff46a73d3d1 in nat44_set_session_limit_command_fn (vm=0x7ff473c8f740,
   input=0x7ff465f36ef0, cmd=0x7ff474c5ce48)
   at /usr/src/debug/vpp-23.02/src/plugins/nat/nat44-ed/nat44_ed_cli.c:1638
14 0x00007ff4b5d56527 in vlib_cli_dispatch_sub_commands (vm=0x7ff473c8f740,
   cm=0x4273f0 <vlib_global_main+48>, input=0x7ff465f36ef0, parent_command_index=97)
   at /usr/src/debug/vpp-23.02/src/vlib/cli.c:650
15 0x00007ff4b5d562c3 in vlib_cli_dispatch_sub_commands (vm=0x7ff473c8f740,
   cm=0x4273f0 <vlib_global_main+48>, input=0x7ff465f36ef0, parent_command_index=98)
   at /usr/src/debug/vpp-23.02/src/vlib/cli.c:607
16 0x00007ff4b5d562c3 in vlib_cli_dispatch_sub_commands (vm=0x7ff473c8f740,
   cm=0x4273f0 <vlib_global_main+48>, input=0x7ff465f36ef0, parent_command_index=21)
   at /usr/src/debug/vpp-23.02/src/vlib/cli.c:607
17 0x00007ff4b5d562c3 in vlib_cli_dispatch_sub_commands (vm=0x7ff473c8f740,
   cm=0x4273f0 <vlib_global_main+48>, input=0x7ff465f36ef0, parent_command_index=0)
   at /usr/src/debug/vpp-23.02/src/vlib/cli.c:607
18 0x00007ff4b5d569cb in vlib_cli_input (vm=0x7ff473c8f740, input=0x7ff465f36ef0,
   function=0x7ff4b5dc2406 <unix_vlib_cli_output>, function_arg=0)
---Type <return> to continue, or q <return> to quit---
   at /usr/src/debug/vpp-23.02/src/vlib/cli.c:753
19 0x00007ff4b5dc7b0c in unix_cli_process_input (cm=0x7ff4b5e4ae00 <unix_cli_main>,
   cli_file_index=0) at /usr/src/debug/vpp-23.02/src/vlib/unix/cli.c:2616
20 0x00007ff4b5dc825a in unix_cli_process (vm=0x7ff473c8f740, rt=0x7ff4797a5280, f=0x0)
   at /usr/src/debug/vpp-23.02/src/vlib/unix/cli.c:2745
21 0x00007ff4b5d80a25 in vlib_process_bootstrap (_a=140687718901968)
   at /usr/src/debug/vpp-23.02/src/vlib/main.c:1221
22 0x00007ff4b439e298 in clib_calljmp () at /usr/src/debug/vpp-23.02/src/vppinfra/longjmp.S:123
23 0x00007ff4698268a0 in ?? ()
24 0x00007ff4b5d80b4e in vlib_process_startup (vm=0x7ff4b43a77a3 <clib_mem_size+24>,
   p=0x7ff4698268d0, f=0x7ff474b1e580) at /usr/src/debug/vpp-23.02/src/vlib/main.c:1246
25 0x00007ff4b5dbdbe6 in vec_max_bytes (v=0x8)
   at /usr/src/debug/vpp-23.02/src/vppinfra/vec_bootstrap.h:161
26 0x00007ff474b1e598 in ?? ()
27 0x0000000000000004 in ?? ()
28 0x00000000000000ff in ?? ()
29 0x00007ff469826980 in ?? ()
30 0x00007ff4b5dbddcb in _vec_set_len (
   v=<error reading variable: Cannot access memory at address 0xfffffffffffffff5>,
   len=<error reading variable: Cannot access memory at address 0xffffffffffffffed>,
   elt_sz=<error reading variable: Cannot access memory at address 0xffffffffffffffe5>)
   at /usr/src/debug/vpp-23.02/src/vppinfra/vec_bootstrap.h:196
Backtrace stopped: previous frame inner to this frame (corrupt stack?)
(gdb)

Type: fix

Signed-off-by: Huawei LI <lihuawei_zzu@163.com>
Change-Id: I52a7d229c95e4ab30f7f2cfe574440aa37bed6a2