rbd: only reset capacity when pointing to head

Snapshots cannot be resized, and the new capacity of head should not
be reflected by the snapshot.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 730d0ce..f171ceb 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -1725,7 +1725,12 @@
 		return ret;
 
 	/* resized? */
-	set_capacity(rbd_dev->disk, h.image_size / SECTOR_SIZE);
+	if (rbd_dev->snap_id == CEPH_NOSNAP) {
+		sector_t size = (sector_t) h.image_size / SECTOR_SIZE;
+
+		dout("setting size to %llu sectors", (unsigned long long) size);
+		set_capacity(rbd_dev->disk, size);
+	}
 
 	down_write(&rbd_dev->header_rwsem);