malloc: fix realloc padded element size

malloc: fix realloc padded element size

When resize a memory with next element, the original element size grows.
If the orginal element has padding, the real inner element size didn't
grow as well and this causes trailer verification failure when malloc
debug enabled.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Xueming Li <xuemingl@mellanox.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>