Magisk 21.0 changed the mount path, we have to get it with "magisk --path" now.
This is used to get where the system partition is mounted.
The phh-su bind-mount is on /system/xbin/su, even with Magisk.
But we still have to unmount it, in order to remove the empty "su" file on the system partition.
With Magisk, depending on enabled modules, directories like /system/xbin, /system/app and others could be tmpfs,
with /system/xbin/su and /system/app/me.phh.superuser being bind-mounts.
So we should try to operate on the real system partition.
