MAINTAINERS: copy virtio on balloon_compaction.c

Changes to mm/balloon_compaction.c can easily break virtio, and virtio
is the only user of that interface.  Add a line to MAINTAINERS so
whoever changes that file remembers to copy us.

Link: http://lkml.kernel.org/r/1501764010-24456-1-git-send-email-mst@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Wei Wang <wei.w.wang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -14004,6 +14004,7 @@ F:	drivers/block/virtio_blk.c
 F:	include/linux/virtio*.h
 F:	include/uapi/linux/virtio_*.h
 F:	drivers/crypto/virtio/
+F:	mm/balloon_compaction.c
 
 VIRTIO CRYPTO DRIVER
 M:	Gonglei <arei.gonglei@huawei.com>