blk-flush: fix invalid BUG_ON in blk_insert_flush

A user reported a regression due to commit
4853abaae7e4a2af938115ce9071ef8684fb7af4 (block: fix flush
machinery for stacking drivers with differring flush flags).
Part of the problem is that blk_insert_flush required a
single bio be attached to the request.  In reality, having
no attached bio is also a valid case, as can be observed with
an empty flush.

[1] http://www.redhat.com/archives/dm-devel/2011-September/msg00154.html

Reported-by: Christophe Saout <christophe@saout.de>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com
Acked-by: Tejun Heo <tj@kernel.org>

Stable note: 3.1
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -320,7 +320,7 @@ void blk_insert_flush(struct request *rq)
 		return;
 	}
 
-	BUG_ON(!rq->bio || rq->bio != rq->biotail);
+	BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */
 
 	/*
 	 * If there's data but flush is not necessary, the request can be