[PATCH] md: Make sure QUEUE_FLAG_CLUSTER is set properly for md.

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>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>