Mempool ops may register in different order when compiling. The default
ops index is always zero but the wanted ops is ring_mp_mc. This patch
sets best mempool ops before generating mbuf pool to fix this issue.
Fixes: 5194299d6ef5 ("examples/ntb: support more functions")
Cc: stable@dpdk.org
Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>