Stale mount on disconnected device: how to delete it?

[Yuri]

I had USB camera connected and recognized as umass0 and mounted as /mnt/camera
on /dev/da0s1.

Camera was disconnected while it was still mounted.

Now 'mount' command shows:
/dev/da0s1 on /mnt/camera (msdosfs, local)

'umount /dev/da0s1' command tells that device is not configured correctly.

'umount -f /dev/da0s1' causes the system to hang.

And when I try to reconnect the camera it shows up as /dev/da1s1.

How to delete stale mount and stale device?

Should I file a PR for this system hang?

I use 7.0-BETA4.

Yuri