Fix bug that made automount(8) never unmount stale autofs(5) mounts,
ie mounts for entries that were there in auto_master(5), and then
got removed.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275756 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f