When taking a snapshot on a UFS/FFS filesystem, it must be mounted.

The "update" mount option must be specified when the "snapshot"
mount option is used. Return EINVAL if the "snapshot" option is
specified without the "update" option also requested.

Reported by: Robert Morris
Reviewed by: kib
PR: 265362
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation