mempool: remove const qualifier when browsing pools

mempool: remove const qualifier when browsing pools

This commit removes the const qualifier for the mempool in
rte_mempool_walk() callback prototype.

Indeed, most functions that can be done on a mempool require a non-const
mempool pointer, except the dump and the audit. Therefore, the
mempool_walk() is more useful if the mempool pointer is not const.

This is required by next commit where the mellanox drivers use
rte_mempool_walk() to iterate the mempools, then rte_mempool_obj_iter()
to iterate the objects in each mempool.

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