udisks-part-id: Shuffle code to keep udev_device in main()

udisks-part-id: Shuffle code to keep udev_device in main()

Reorder the code to get and keep an udev_device struct in main() and pass it
down to get_part_table_device_file(), instead of temporarily creating them in
that function and get_syspath(). This avoids duplicate lookups and also
allows us to do further queries on the udev_device later on (which we need to
fix bz#26996)