Legacy mountpoint datasets should not pass `-o zfsutil` to `mount.zfs`.
Fix the logic in `mount_fs()` to not forget we have a legacy mountpoint
when checking for an `org.zol:mountpoint` userprop.
Reviewed-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#14274