Scan also 'mount' output for mounted filesystem.

Scan also 'mount' output for mounted filesystem.

As util-linux package seems to give all the time different names,
try harder to figure out, where is the given lv possible mounted
and scan /proc/mounts and if not found there, test also 'mount' output.

/dev/dm-xxx
/dev/mapper/vg-lv
/dev/vg/lv

All of them could be used different combination in /proc/mount and mount output.

Patch fixes regression for older systems where new detection code failed to
find valid combination.