Looking for directories in `/sys/fs` is not exhaustive. Not all available
filesystems are listed there.

A library function has been already developed for such a use case. It is
`load_fstype``.

This patch replaces custom code with a call to `load_fstype`

Signed-off-by: Federico Vaga <[email protected]>

The ISO checksum code was executed independently of the provided
`$livedev`. Often, this is a loop device pointing to an ISO image, but
in other cases `dmsquash-live-root` receives the path to a filesystem
image. In this case, we can't use `udevadm` to extract information
because it is not a device, and trying to do that leads to `udevadm`
error messages (but not blocking).

Therefore, the ISO checksum check must be performed only if the provided
`$livedev` is **not** a regular file.

Signed-off-by: Federico Vaga <[email protected]>