Fix several problems in linux_find_root_device

'stat' returns the major and minor numbers in hexadecimal, while
'mountpoint' returns them in decimal; convert the ones from 'stat' so
comparison actually work.

Rename variables to reflect better the difference between the DEVICE
representation (major:minor) and the NODE representation (a path of a
file under /dev).

Finally fix the invocation of linux_find_root_device as well.