mempool: create internal ring in a specific function

mempool: create internal ring in a specific function

This makes the code of rte_mempool_create() clearer, and it will make
the introduction of external mempool handler easier (in another patch
series). Indeed, this function contains the specific part when a ring is
used, but it could be replaced by something else in the future.

This commit also adds a socket_id field in the mempool structure that
is used by this new function.

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