mempool: support no hugepage mode

mempool: support no hugepage mode

Introduce a new function rte_mempool_populate_virt() that is now called
by default when hugepages are not supported. This function populate the
mempool with several physically contiguous chunks whose minimum size is
the page size of the system.

Thanks to this, rte_mempool_create() will work properly in without
hugepages (if the object size is smaller than a page size), and 2
specific workarouds can be removed:

- trailer_size was artificially extended to a page size
- rte_mempool_virt2phy() did not rely on object physical address

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