[PATCH] dm: make sure QUEUE_FLAG_CLUSTER is set properly

This flag should be set for a virtual device iff it is set for all
underlying devices.

Signed-off-by: Neil Brown <neilb@suse.de>
Acked-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>