rbd: preserve snapc->seq in rbd_header_set_snap()

In rbd_header_set_snap(), there is logic to make the snap context's
seq field get set to a particular snapshot id, or 0 if there is no
snapshot for the rbd image.

This seems to be an artifact of how the current snapshot id for an
rbd_dev was recorded before the rbd_dev->snap_id field began to be
used for that purpose.

There's no need to update the value of snapc->seq here any more, so
stop doing it.  Tidy up a few local variables in that function
while we're at it.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -588,29 +588,25 @@ static int snap_by_name(struct rbd_image_header *header, const char *snap_name,
 
 static int rbd_header_set_snap(struct rbd_device *rbd_dev, u64 *size)
 {
-	struct rbd_image_header *header = &rbd_dev->header;
-	struct ceph_snap_context *snapc = header->snapc;
-	int ret = -ENOENT;
+	int ret;
 
 	down_write(&rbd_dev->header_rwsem);
 
 	if (!memcmp(rbd_dev->snap_name, RBD_SNAP_HEAD_NAME,
 		    sizeof (RBD_SNAP_HEAD_NAME))) {
-		if (header->total_snaps)
-			snapc->seq = header->snap_seq;
-		else
-			snapc->seq = 0;
 		rbd_dev->snap_id = CEPH_NOSNAP;
 		rbd_dev->snap_exists = false;
 		rbd_dev->read_only = 0;
 		if (size)
-			*size = header->image_size;
+			*size = rbd_dev->header.image_size;
 	} else {
-		ret = snap_by_name(header, rbd_dev->snap_name,
-					&snapc->seq, size);
+		u64 snap_id = 0;
+
+		ret = snap_by_name(&rbd_dev->header, rbd_dev->snap_name,
+					&snap_id, size);
 		if (ret < 0)
 			goto done;
-		rbd_dev->snap_id = snapc->seq;
+		rbd_dev->snap_id = snap_id;
 		rbd_dev->snap_exists = true;
 		rbd_dev->read_only = 1;
 	}