automount(8): when flushing autofs, specify fsid
Which should avoid situation where flushed filesystem is not autofs,
because it was mounted over autofs mp.
Reported and tested by: Andrew "RhodiumToad" Gierth <andrew@tao11.riddles.org.uk>
PR: 272446
Reviewed by: mjg
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42023