Use bd_claim_by_disk.

Following symlinks are created if dm-0 maps to sda:
  /sys/block/dm-0/slaves/sda --> /sys/block/sda
  /sys/block/sda/holders/dm-0 --> /sys/block/dm-0
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Cc: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>