Btrfs: fix subvol_name leak on error in btrfs_mount()

btrfs_parse_early_options() can fail due to error while scanning devices
(-o device= option), but still strdup() subvol_name string:

mount -o subvol=SUBV,device=BAD_DEVICE <dev> <mnt>

So free subvol_name string on error.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/btrfs/super.c

@@ -905,8 +905,10 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	error = btrfs_parse_early_options(data, mode, fs_type,
 					  &subvol_name, &subvol_objectid,
 					  &subvol_rootid, &fs_devices);
-	if (error)
+	if (error) {
+		kfree(subvol_name);
 		return ERR_PTR(error);
+	}
 
 	if (subvol_name) {
 		root = mount_subvol(subvol_name, flags, device_name, data);