l2: mark l2 fib uninitialized after clearing

l2: mark l2 fib uninitialized after clearing

Type: fix

After clearing the l2fib with 'vppctl clear l2fib', a SEGV occurs the
next time a MAC address is learned.

In l2fib_clear_table(), the l2fib bihash is freed and then
l2fib_table_init() is called to reinitialize it. l2fib_table_init()
will not do anything if l2fib_main.mac_table_initialized is set to 1.
Reset the value of l2fib_main.mac_table_initialized to 0 before
calling l2fib_table_init().

Change-Id: I87f1a3f9a46c951f36c1c0a5ab795b0ec08c81a8
Signed-off-by: Matthew Smith <mgsmith@netgate.com>