rbd: fix error handling from rbd_snap_name()

rbd_snap_name() calls rbd_dev_v{1,2}_snap_name() depending on the
format of the image. The format 1 version returns NULL on error, which
is handled by the caller. The format 2 version returns an ERR_PTR,
which the caller of rbd_snap_name() does not expect.

Fortunately this is unlikely to occur in practice because
rbd_snap_id_by_name() is called before rbd_snap_name(). This would hit
similar errors to rbd_snap_name() (like the snapshot not existing) and
return early, so rbd_snap_name() would not hit an error unless the
snapshot was removed between the two calls or memory was exhausted.

Use an ERR_PTR in rbd_dev_v1_snap_name() so that the specific error
can be propagated, and it is consistent with rbd_dev_v2_snap_name().
Handle the ERR_PTR in the only rbd_snap_name() caller.

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

drivers/block/rbd.c

@@ -927,12 +927,14 @@ static const char *rbd_dev_v1_snap_name(struct rbd_device *rbd_dev,
 					u64 snap_id)
 {
 	u32 which;
+	const char *snap_name;
 
 	which = rbd_dev_snap_index(rbd_dev, snap_id);
 	if (which == BAD_SNAP_INDEX)
-		return NULL;
+		return ERR_PTR(-ENOENT);
 
-	return _rbd_dev_v1_snap_name(rbd_dev, which);
+	snap_name = _rbd_dev_v1_snap_name(rbd_dev, which);
+	return snap_name ? snap_name : ERR_PTR(-ENOMEM);
 }
 
 static const char *rbd_snap_name(struct rbd_device *rbd_dev, u64 snap_id)
@@ -4163,8 +4165,8 @@ static int rbd_dev_spec_update(struct rbd_device *rbd_dev)
 	/* Look up the snapshot name, and make a copy */
 
 	snap_name = rbd_snap_name(rbd_dev, spec->snap_id);
-	if (!snap_name) {
-		ret = -ENOMEM;
+	if (IS_ERR(snap_name)) {
+		ret = PTR_ERR(snap_name);
 		goto out_err;
 	}