rbd: have snap_by_name() return a snapshot

A function called snap_by_name() ought to just look up a snapshot by
name.  It does that, but then it assigns some stuff to the rbd
device structure as well.

Change the function to do just the lookup, and have the caller do
the assignments that follow.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -830,44 +830,39 @@ static const char *rbd_snap_name(struct rbd_device *rbd_dev, u64 snap_id)
 	return NULL;
 }
 
-static int snap_by_name(struct rbd_device *rbd_dev, const char *snap_name)
+static struct rbd_snap *snap_by_name(struct rbd_device *rbd_dev,
+					const char *snap_name)
 {
-
 	struct rbd_snap *snap;
 
-	list_for_each_entry(snap, &rbd_dev->snaps, node) {
-		if (!strcmp(snap_name, snap->name)) {
-			rbd_dev->spec->snap_id = snap->id;
-			rbd_dev->mapping.size = snap->size;
-			rbd_dev->mapping.features = snap->features;
-
-			return 0;
-		}
-	}
+	list_for_each_entry(snap, &rbd_dev->snaps, node)
+		if (!strcmp(snap_name, snap->name))
+			return snap;
 
-	return -ENOENT;
+	return NULL;
 }
 
 static int rbd_dev_set_mapping(struct rbd_device *rbd_dev)
 {
-	int ret;
-
 	if (!memcmp(rbd_dev->spec->snap_name, RBD_SNAP_HEAD_NAME,
 		    sizeof (RBD_SNAP_HEAD_NAME))) {
 		rbd_dev->spec->snap_id = CEPH_NOSNAP;
 		rbd_dev->mapping.size = rbd_dev->header.image_size;
 		rbd_dev->mapping.features = rbd_dev->header.features;
-		ret = 0;
 	} else {
-		ret = snap_by_name(rbd_dev, rbd_dev->spec->snap_name);
-		if (ret < 0)
-			goto done;
+		struct rbd_snap *snap;
+
+		snap = snap_by_name(rbd_dev, rbd_dev->spec->snap_name);
+		if (!snap)
+			return -ENOENT;
+		rbd_dev->spec->snap_id = snap->id;
+		rbd_dev->mapping.size = snap->size;
+		rbd_dev->mapping.features = snap->features;
 		rbd_dev->mapping.read_only = true;
 	}
 	set_bit(RBD_DEV_FLAG_EXISTS, &rbd_dev->flags);
 
-done:
-	return ret;
+	return 0;
 }
 
 static void rbd_header_free(struct rbd_image_header *header)