app/testpmd: fix multicast address pool leak

app/testpmd: fix multicast address pool leak

A multicast address pool is allocated for a port when
using mcast_addr testpmd commands.

When closing a port or stopping testpmd, this pool was
not freed, resulting in a leak.
This issue has been caught using ASan.

Free this pool when closing the port.

Error info as following:
ERROR: LeakSanitizer: detected memory leaksDirect leak of
       192 byte(s)
0 0x7f6a2e0aeffe in __interceptor_realloc
	(/lib/x86_64-linux-gnu/libasan.so.5+0x10dffe)
1 0x565361eb340f in mcast_addr_pool_extend
	../app/test-pmd/config.c:5162
2 0x565361eb3556 in mcast_addr_pool_append
	../app/test-pmd/config.c:5180
3 0x565361eb3aae in mcast_addr_add
	../app/test-pmd/config.c:5243

Fixes: 8fff667578a7 ("app/testpmd: new command to add/remove multicast MAC addresses")
Cc: stable@dpdk.org
Signed-off-by: Ke Zhang <ke1x.zhang@intel.com>
Acked-by: Yuying Zhang <yuying.zhang@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@xilinx.com>