In some places we want to read a single u64 value from a sysfs path, or
from fsid directory. Add helpers that do that in one go.
Signed-off-by: David Sterba <dsterba@suse.com>
The sysfs could use more convenience helpers so move the current code to
own file before adding more helpers.
Signed-off-by: David Sterba <dsterba@suse.com>