mempool: remove const qualifier in dump and audit

mempool: remove const qualifier in dump and audit

In next commits, we will use an iterator to walk through the objects in
mempool in rte_mempool_audit(). This iterator takes a "struct
rte_mempool *" as a parameter because it is assumed that the callback
function can modify the mempool.

The previous approach was to introduce a RTE_DECONST() macro, but
after discussion it seems that removing the const qualifier is better
to avoid fooling the compiler, and also because these functions are
not used in datapath (possible compiler optimizations due to const
are not critical).

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>