fsadm: if available, use /proc/self/mountinfo to detect mounted volume

fsadm: if available, use /proc/self/mountinfo to detect mounted volume

The /proc/self/mountinfo is not bound to device names like /proc/mounts
and it uses major:minor pairs instead.

This fixes a situation in which a volume is mounted and then renamed
later on - that makes /proc/mounts unreliable when detecting mounted
volumes.

See also https://bugzilla.redhat.com/show_bug.cgi?id=1196910.