Revert "initramfs: use mount.zfs instead of mount"

This broke mounting of snapshots on / for users.

See https://github.com/openzfs/zfs/issues/9461#issuecomment-1376162949 for more context.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #14908