LP: #1175406 Avoid possible reference to /dev/dm-*

Or just UUID=$uuid instead of $(findfs UUID=$uuid)?

After installing Kubuntu 13.04 I moved some partitions and drives around, and crypttab pointed to /dev/dm-3, which got re-numbered at some point to /dev/vg/home instead of vg/swap. So eventually it clobbered that volume when it swapped something. I think this may help.