mem: fix malloc element free in debug mode

mem: fix malloc element free in debug mode

malloc_elem_free() is clearing(setting to 0) the trailer cookie when
RTE_MALLOC_DEBUG is enabled. In case of joining free neighbor element,
part of joined memory is not getting cleared due to missing the length
of trailer cookie in the middle.

This patch fixes calculation of free memory length to be cleared in
malloc_elem_free() by including trailer cookie.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Xueming Li <xuemingl@mellanox.com>
Acked-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>