mempool: rename object constructor typedef

mempool: rename object constructor typedef

This commit renames mempool_obj_ctor_t as mempool_obj_cb_t.

In next commits, we will add the ability to populate the
mempool and iterate through objects using the same function.
We will use the same callback type for that. As the callback is
not a constructor anymore, rename it into rte_mempool_obj_cb_t.

The rte_mempool_obj_iter_t that was used to iterate over objects
will be removed in next commits.

No functional change.
In this commit, the API is preserved through a compat typedef.

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