btrfs: add missing mutex_unlock in btrfs_relocate_sys_chunks()

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%2fHpAGFqa7YAFuM@duo.ucw.cz
Reported-by: Pavel Machek <pavel@denx.de>
Fixes: 7411055 ("btrfs: handle chunk tree lookup error in btrfs_relocate_sys_chunks()")
CC: stable@vger.kernel.org
Reviewed-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -3419,6 +3419,7 @@ static int btrfs_relocate_sys_chunks(struct btrfs_fs_info *fs_info)
 			 * alignment and size).
 			 */
 			ret = -EUCLEAN;
+			mutex_unlock(&fs_info->reclaim_bgs_lock);
 			goto error;
 		}