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: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>
        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();