virtio-balloon: fix a typo in comment of virtballoon_migratepage()

Typo: compation --> compaction

Signed-off-by: Liu Xiang <liu.xiang@zlingsmart.com>
Link: https://lore.kernel.org/r/20210327031710.16151-1-liu.xiang@zlingsmart.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>