mempool: introduce a function to free a pool

Introduce rte_mempool_free() that:

- unlink the mempool from the global list if it is found
- free all the memory chunks using their free callbacks
- free the internal ring
- free the memzone containing the mempool

Currently this function is only used in error cases when
creating a new mempool, but it will be made public later
in the patch series.

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