initramfs: Make mountpoint=none work

In initramfs, mount.zfs fails to mount a dataset with mountpoint=none,
but mount.zfs -o zfsutil works. Use -o zfsutil when mountpoint=none.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #14455