losetup: add --loop-ref and REF column

losetup: add --loop-ref and REF column

The lo_file_name is nowhere used (kernel uses backing file descriptor,
no path) and it was used to store limited info about the backing file path
(64 bytes only!). For backward compatibility, we still fill lo_file_name
with the path, but it's nowhere in the userspace used as the complete
backing file path in sysfs.

This commit introduces a new option to overwrite the default path in
lo_file_name. The idea is to use the reference string by udevd in
/dev/loop/by-ref to address loop devices independently on paths.

Addresses: https://github.com/util-linux/util-linux/issues/2106
Suggested-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Karel Zak <kzak@redhat.com>