test/table: fix build with GCC 11

test/table: fix build with GCC 11

Build error:
../app/test/test_table_tables.c: In function ‘test_table_stub’:
../app/test/test_table_tables.c:31:9:
	warning: ‘memset’ offset [0, 31] is out of the bounds [0, 0]
	[-Warray-bounds]
         memset((uint8_t *)mbuf + sizeof(struct rte_mbuf) + 32, 0, 32); \
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../app/test/test_table_tables.c:151:25:
	note: in expansion of macro ‘PREPARE_PACKET’
  151 |                         PREPARE_PACKET(mbufs[i], 0xadadadad);
      |                         ^~~~~~~~~~~~~~

'key' points to mbuf header + 32 bytes, and memset clears next 32 bytes
of 'key', so overall there needs to be 64 bytes after mbuf header.
Adding a mbuf size check before memset.

The original code has an assumption that mbuf data buffer follows mbuf
header, this patch accepts same assumption.

Bugzilla ID: 677
Fixes: 5205954791cb ("app/test: packet framework unit tests")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>