initramfs: Fix legacy mountpoint rootfs

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