eal: fix default mempool ops

eal: fix default mempool ops

If '--mbuf-pool-ops' is not passed to EAL as command line argument then
rte_eal_mbuf_default_mempool_ops will return NULL.

Instead check if internal_config.user_mbuf_pool_ops_name is NULL and
return compile time RTE_MBUF_DEFAULT_MEMPOOL_OPS.

Fixes: 8b0f7f43413 ("mbuf: maintain user and compile time mempool ops name")

Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>