mempool: use the list to initialize objects

mempool: use the list to initialize objects

Before this patch, the mempool elements were initialized at the time
they were added to the mempool. This patch changes this to do the
initialization of all objects once the mempool is populated, using
rte_mempool_obj_iter() introduced in previous commits.

Thanks to this modification, we are getting closer to a new API
that would allow us to do:
  mempool_init()
  mempool_populate(mem1)
  mempool_populate(mem2)
  mempool_populate(mem3)
  mempool_init_obj()

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