fs/btrfs/volumes.c: add missing free_fs_devices

Free fs_devices as done in the error-handling code just below.
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>

fs/btrfs/volumes.c

@@ -4352,8 +4352,10 @@ static int open_seed_devices(struct btrfs_root *root, u8 *fsid)
 
 	ret = __btrfs_open_devices(fs_devices, FMODE_READ,
 				   root->fs_info->bdev_holder);
-	if (ret)
+	if (ret) {
+		free_fs_devices(fs_devices);
 		goto out;
+	}
 
 	if (!fs_devices->seeding) {
 		__btrfs_close_devices(fs_devices);