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.

I submitted this late at night. Now in the morning another solution
came to my mind. I thought I will find it in replies but I didn't.

In case of USB device (which device in question in this problem
happens to be) usbd can be used to mount it.

If attach/detach events trigger mount/unmount commands this problem
shouldn't exist. I didn't try though.

Otherwise for non-USB devices this situation probably can only happen
in case of failing devices.

Yuri