mm/balloon_compaction: suppress allocation warnings

There is no reason to print warnings when balloon page allocation fails,
as they are expected and can be handled gracefully.  Since VMware
balloon now uses balloon-compaction infrastructure, and suppressed these
warnings before, it is also beneficial to suppress these warnings to
keep the same behavior that the balloon had before.

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>