mempool: create the internal ring when populating

mempool: create the internal ring when populating

Instead of creating the internal ring at mempool creation, do
it when populating the mempool with the first memory chunk. The
objective here is to simplify the change of external handler
when it will be introduced.

For instance, this will be possible:

  mp = rte_mempool_create_empty(...)
  rte_mempool_set_ext_handler(mp, my_handler)
  rte_mempool_populate_default()

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