[PATCH] device-mapper: add dm_find_md

Abstract dm_find_md() from dm_get_mdptr() to allow use elsewhere.

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>