mbuf: introduce a new helper to create a pool

mbuf: introduce a new helper to create a pool

Add a new wrapper to rte_mempool_create() to simplify the creation
of a packet mbuf pool.

This wrapper can be used if there is no specific mempool flags, and
no specific mbuf or pool constructor function, which is most of the
use cases.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>