Fix remounting ZFS filesystem with "zfs mount"

"zfs mount -o" passes a list of mount options directly to nmount(2) after
sanity checking them. In particular, zfs(8) will refuse to mount an already
existing file system unless "remount" is specified in the option list.
However, the "remount" option only exists in Illumos. FreeBSD's equivalent is
"update".

PR: 221985
Reviewed by: avg
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12233