Fix panic that would happen on forcibly unmounting devfs (note that
as it is now, devfs ignores MNT_FORCE anyway, so it needs to be modified
to trigger the panic) with consumers still opened.

Note that this still results in a leak of r/w/e counters. It seems
to be harmless, though. If anyone knows a better way to approach
this - please tell.

Discussed with: kib@, mav@
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3050