Match full volume sysfs instead of prefix.

So dm-10 doesn't get mistaken for dm-1.  I guess this was intentional,
to do with partitions.  I think you could instead list a bunch of
partition numbers you expect to see and get the same effect, but I
don't need them so I haven't tested.

Change-Id: I5412520fa9ad0ce0838fa386af1f33554fd12454