Bug#688336: os-probe: subvol: patches to provide bootloaders with all the subvolume info.

[Mike Mestnik <cheako@mikemestnik.net>]

Using these patches reveals they are not complete, there are two areas
that need to be addressed.

Firstly the kernel can and will use any device in a filesystem for the
contents of /proc/mount.  This causes problems in multiple areas that
os-probe will have to compensate for.

Second 50mounted-tests was not updated at all, so these patches only
work if the subvolume is mounted and you don't encounter the first
issue.