virtio-mem: prepare fake page onlining code for granularity smaller than MAX_ORDER - 1

Let's prepare our fake page onlining code for subblock size smaller than
MAX_ORDER - 1: we might get called for ranges not covering properly
aligned MAX_ORDER - 1 pages. We have to detect the order to use
dynamically.

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20211126134209.17332-3-david@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Eric Ren <renzhengeek@gmail.com>