Btrfs: properly clean up btrfs_end_io_wq_cache

In one of Dave's cleanup commits he forgot to call btrfs_end_io_wq_exit on
unload, which makes us unable to unload and then re-load the btrfs module.  This
fixes the problem.  Thanks,
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Reviewed-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/super.c

@@ -2151,6 +2151,7 @@ static void __exit exit_btrfs_fs(void)
 	extent_map_exit();
 	extent_io_exit();
 	btrfs_interface_exit();
+	btrfs_end_io_wq_exit();
 	unregister_filesystem(&btrfs_fs_type);
 	btrfs_exit_sysfs();
 	btrfs_cleanup_fs_uuids();