mempool: use the list to iterate the elements

mempool: use the list to iterate the elements

Now that the mempool objects are chained into a list, we can use it to
browse them. This implies a rework of rte_mempool_obj_iter() API, that
does not need to take as many arguments as before. The previous function
is kept as a private function, and renamed in this commit. It will be
removed in a next commit of the patch series.

The only internal users of this function are the mellanox drivers. The
code is updated accordingly.

Introducing an API compatibility for this function has been considered,
but it is not easy to do without keeping the old code, as the previous
function could also be used to browse elements that were not added in a
mempool. Moreover, the API is already be broken by other patches in this
version.

The library version was already updated in
commit 213af31e0960 ("mempool: reduce structure size if no cache needed")

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