block: Initial support for data-less (or empty) barrier support

This implements functionality to pass down or insert a barrier
in a queue, without having data attached to it. The ->prepare_flush_fn()
infrastructure from data barriers are reused to provide this
functionality.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

mm/bounce.c

@@ -264,6 +264,12 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
 {
 	mempool_t *pool;
 
+	/*
+	 * Data-less bio, nothing to bounce
+	 */
+	if (bio_empty_barrier(*bio_orig))
+		return;
+
 	/*
 	 * for non-isa bounce case, just check if the bounce pfn is equal
 	 * to or bigger than the highest pfn in the system -- in that case,