mem: revert reversed allocation

mem: revert reversed allocation

A few regressions with virtio/vhost have been discovered, due to the
strong dependency of virtio/vhost on the underlying memory layout.
Specifically, virtio/vhost share all memory pages starting from the
beginning of the segment, while the patch below made it so that the
memory is always allocated from the top of VA space, not from the
bottom.

Fixes: 179f916e88e4 ("mem: allocate in reverse to reduce fragmentation")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>