mbuf: maintain user and compile time mempool ops name

mbuf: maintain user and compile time mempool ops name

At present the userdefined mempool ops name overwrites
the default mempool ops name variable in internal_config.

This patch change the logic to maintain the value of
user defined only in the internal config.

The pktmbuf_create_pool is updated to reflect the same ie.
use user defined. If not present than use the default.

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