Currently userspace code using the dm ioctls must refer to a mapped device by
either its name or its uuid.  But in some circumstances you know neither of
those directly.

This patch lets you reference devices by their major/minor numbers as an
alternative.
Signed-Off-By: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>