mempool: silence warning on pointer arithmetic

Translating from a mempool object to the mempool pointer does not break
alignment constraints.  However, the compiler is unaware of this fact and
complains on -Wcast-align.  This patch modifies the code to use RTE_PTR_SUB(),
thereby silencing the compiler by casting through (void *).

Signed-off-by: Cyril Chemparathy <cchemparathy@ezchip.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>