netdev-dpdk: Remove use of rte_mempool_ops_get_count.

netdev-dpdk: Remove use of rte_mempool_ops_get_count.

rte_mempool_ops_get_count is not exported by DPDK so it means it
cannot be used by OVS when using DPDK as a shared library.

Remove rte_mempool_ops_get_count but still use rte_mempool_full
and document it's behavior.

Fixes: 91fccdad72a2 ("netdev-dpdk: Free mempool only when no in-use mbufs.")
Reported-by: Timothy Redaelli <tredaelli@redhat.com>
Reported-by: Markos Chandras <mchandras@suse.de>
Signed-off-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>