MFC r286237:

MFC r286237:

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.

Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@289511 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f