block: fix failfast merge testing in elv_rq_merge_ok()

Commit ab0fd1de tries to prevent merge
of requests with different failfast settings.  In elv_rq_merge_ok(),
it compares new bio's failfast flags against the merge target
request's.  However, the flag testing accessors for bio and blk don't
return boolean but the tested bit value directly and FAILFAST on bio
and blk don't match, so directly comparing them with == results in
false negative unnecessary preventing merge of readahead requests.

This patch convert the results to boolean by negating them before
comparison.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Boaz Harrosh <bharrosh@panasas.com>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jeff Garzik <jeff@garzik.org>

block/elevator.c

@@ -101,11 +101,16 @@ int elv_rq_merge_ok(struct request *rq, struct bio *bio)
 		return 0;
 
 	/*
-	 * Don't merge if failfast settings don't match
+	 * Don't merge if failfast settings don't match.
+	 *
+	 * FIXME: The negation in front of each condition is necessary
+	 * because bio and request flags use different bit positions
+	 * and the accessors return those bits directly.  This
+	 * ugliness will soon go away.
 	 */
-	if (bio_failfast_dev(bio)	!= blk_failfast_dev(rq)		||
-	    bio_failfast_transport(bio)	!= blk_failfast_transport(rq)	||
-	    bio_failfast_driver(bio)	!= blk_failfast_driver(rq))
+	if (!bio_failfast_dev(bio)	 != !blk_failfast_dev(rq)	||
+	    !bio_failfast_transport(bio) != !blk_failfast_transport(rq)	||
+	    !bio_failfast_driver(bio)	 != !blk_failfast_driver(rq))
 		return 0;
 
 	if (!elv_iosched_allow_merge(rq, bio))