mempool: fix pages computation to determine number of objects

mempool: fix pages computation to determine number of objects

In rte_mempool_obj_iter(), when element boundary coincides with page boundary,
even if a single page is required per object, a loop checks that the next page
is contiguous and drops the first one otherwise.
This commit checks subsequent pages only when several are required per object.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Reviewed-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>