fib: fix ip drop path crashes

fib: fix ip drop path crashes

Do not mark drop paths as imported to avoid crashes on invalid table lookup.

```
vpp[8478]: /build/Vpp2310/source/src/vnet/fib/fib_table.c:35 (fib_table_get) assertion `! pool_is_free (ip4_main.fibs, _e)' fails
 #9  0x00007ff21785da1d in _clib_error () from /lib/x86_64-linux-gnu/libvppinfra.so.23.10
 #10 0x00007ff218087698 in fib_table_get (index=4294967295, proto=FIB_PROTOCOL_IP4) at /build/Vpp2310/source/src/vnet/fib/fib_table.c:35
 #11 0x00007ff218087a37 in fib_table_lookup_exact_match (fib_index=4294967295, prefix=0x7ff0eae0d354) at /build/Vpp2310/source/src/vnet/fib/fib_table.c:100
 #12 0x00007ff2180bc938 in fib_attached_export_import (fib_entry=0x7ff0eceac3e0, export_fib=4294967295) at /build/Vpp2310/source/src/vnet/fib/fib_attached_export.c:264
 #13 0x00007ff218098ade in fib_entry_post_flag_update_actions (fib_entry=0x7ff0eceac3e0, old_flags=FIB_ENTRY_FLAG_NONE, new_fib_index=4294967295) at /build/Vpp2310/source/src/vnet/fib/fib_entry.c:624
 #14 0x00007ff218098b90 in fib_entry_post_install_actions (fib_entry=0x7ff0eceac3e0, source=FIB_SOURCE_API, old_flags=FIB_ENTRY_FLAG_NONE) at /build/Vpp2310/source/src/vnet/fib/fib_entry.c:674
 #15 0x00007ff218098cce in fib_entry_create (fib_index=1, prefix=0x7ff0d3244d80, source=FIB_SOURCE_API, flags=FIB_ENTRY_FLAG_NONE, paths=0x7ff0eac15ab8) at /build/Vpp2310/source/src/vnet/fib/fib_entry.c:712
 #16 0x00007ff218088db4 in fib_table_entry_update (fib_index=1, prefix=0x7ff0d3244d80, source=FIB_SOURCE_API, flags=FIB_ENTRY_FLAG_NONE, paths=0x7ff0eac15ab8) at /build/Vpp2310/source/src/vnet/fib/fib_table.c:799
 #17 0x00007ff2180c026c in fib_api_route_add_del (is_add=1 '\001', is_multipath=0 '\000', fib_index=1, prefix=0x7ff0d3244d80, src=FIB_SOURCE_API, entry_flags=FIB_ENTRY_FLAG_NONE, rpaths=0x7ff0eac15ab8) at /build/Vpp2310/source/src/vnet/fib/fib_api.c:485
 #18 0x00007ff217d4b6dd in ip_route_add_del_t_handler (mp=0x7ff0eb08b998, stats_index=0x7ff0d3244dc8) at /build/Vpp2310/source/src/vnet/ip/ip_api.c:718
 #19 0x00007ff217d4b986 in vl_api_ip_route_add_del_t_handler (mp=0x7ff0eb08b998) at /build/Vpp2310/source/src/vnet/ip/ip_api.c:789
```

Type: fix
Fixes: 4b08632748727486e7ebfdcf4d992743595bc500
Signed-off-by: Dmitry Valter <[email protected]>
Change-Id: I647899533771c35f44c9ecde517a30f111b36ad9