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>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index ce289af..3b98417 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4352,8 +4352,10 @@
 
 	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);