hung during bcachefs umount

[Hongbo Li]

Hi, Kent,
I found the latest repo( 
69558c638c465a79be3a08bfeb3d5a15979cbe42(bcachefs: fix ei_update_lock 
lock ordering)) on master branch will cause stucking during bcachefs umount.

Here is the test step:

```
mount -t bcachefs /dev/loop1 /mnt/bcachefs
umount /mnt/bcachefs                      ---- stuck here !!!
```

Thanks,
Hongbo