mbuf: use pktmbuf helper to create the pool

mbuf: use pktmbuf helper to create the pool

When possible, replace the uses of rte_mempool_create() with
the helper provided in librte_mbuf: rte_pktmbuf_pool_create().

This is the preferred way to create a mbuf pool.

This also updates the documentation.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Signed-off-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>