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

Let's prepare our page onlining code for subblock size smaller than
MAX_ORDER - 1: we'll get called for a MAX_ORDER - 1 page but might have
some subblocks in the range plugged and some unplugged. In that case,
fallback to subblock granularity to properly only expose the plugged
parts to the buddy.

Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20211126134209.17332-2-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>