mempool: rename functions with confusing names

mempool: rename functions with confusing names

The mempool_count and mempool_free_count behaved contrary to what their
names suggested. The free_count function actually returned the number of
elements that were allocated from the pool, not the number unallocated as
the name implied.

Fix this by introducing two new functions to replace the old ones,
* rte_mempool_avail_count to replace rte_mempool_count
* rte_mempool_in_use_count to replace rte_mempool_free_count

In this patch, the new functions are added, and the old ones are marked
as deprecated. All apps and examples that use the old functions are
updated to use the new functions.

Fixes: af75078fece3 ("first public release")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>