btrfs: add missing mutex_unlock in btrfs_relocate_sys_chunks()

commit 9af503d upstream.

The previous patch that replaced BUG_ON by error handling forgot to
unlock the mutex in the error path.

Link: https://lore.kernel.org/all/Zh%[email protected]
Reported-by: Pavel Machek <[email protected]>
Fixes: 7411055 ("btrfs: handle chunk tree lookup error in btrfs_relocate_sys_chunks()")
CC: [email protected]
Reviewed-by: Pavel Machek <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/btrfs/volumes.c

@@ -3277,6 +3277,7 @@ static int btrfs_relocate_sys_chunks(struct btrfs_fs_info *fs_info)
 			 * alignment and size).
 			 */
 			ret = -EUCLEAN;
+			mutex_unlock(&fs_info->delete_unused_bgs_mutex);
 			goto error;
 		}