mem: introduce memzone freeing

mem: introduce memzone freeing

Implement rte_memzone_free which, as its name implies, would free a
memzone.

Currently memzone are tracked in an array and cannot be free.
To be able to reuse the same array to track memzones, we have to
change how we keep track of reserved memzones.

With this patch, any memzone with addr NULL is not used, so we also need
to change how we look for the next memzone entry free.

Add new unit test for rte_memzone_free API.

Signed-off-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>