Replace use of the newly-deprecated rte_mempool_count() with the new
name, rte_mempool_avail_count().
Also add a compatibility wrapper so that builds against older DPDK
versions still work.
Change-Id: If3c44bdef4bbcf7a456a1dfa272348ccc6f35261
Reported-by: Jay Sternberg <jay.e.sternberg@intel.com>
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>