mempool: introduce a free callback for memory chunks

mempool: introduce a free callback for memory chunks

Introduce a free callback that is passed to the populate* functions,
which is used when freeing a mempool. This is unused now, but as next
commits will populate the mempool with several chunks of memory, we
need a way to free them properly on error.

Later in the series, we will also introduce a public rte_mempool_free()
and the ability for the user to populate a mempool with its own memory.
For that, we also need a free callback.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>