snapshot-hook should be more flexible about device names

[ewxrjk]

After upgrading to Debian jessie, df reports my root LV as /dev/dm-0 rather than /dev/mapper/something, causing the snapshot hook to fail. Ideally it should recognize any name for a logical volume, regardless of links, bind mounts or any other kind of aliasing.

[ewxrjk]

09:36 < ewx> is there a standard way of recovering a name that will be 
             acceptable to LVM from an alias for the device?  i.e. turning 
             /dev/dm-0 into /dev/mapper/whatever-whatever
09:39 < col> I'd be inclined to use something involving udevadm info -n 
             DEVICE-NODE
09:41 < ewx> that looks promising, thanks.

[ewxrjk]

You will also need 7b84f91, the original versions was broken despite the tests. (I don't have a non-production LVM test rig.)