block: rename bio bi_rw to bi_opf

Since commit 63a4cc24867d, bio->bi_rw contains flags in the lower
portion and the op code in the higher portions. This means that
old code that relies on manually setting bi_rw is most likely
going to be broken. Instead of letting that brokeness linger,
rename the member, to force old and out-of-tree code to break
at compile time instead of at runtime.

No intended functional changes in this commit.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 2c3ab6f..d646f6e 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -285,7 +285,7 @@
 	 */
 	sectors = bio_sectors(bio);
 	/* bio could be mergeable after passing to underlayer */
-	bio->bi_rw &= ~REQ_NOMERGE;
+	bio->bi_opf &= ~REQ_NOMERGE;
 	mddev->pers->make_request(mddev, bio);
 
 	cpu = part_stat_lock();
@@ -414,7 +414,7 @@
 		/* an empty barrier - all done */
 		bio_endio(bio);
 	else {
-		bio->bi_rw &= ~REQ_PREFLUSH;
+		bio->bi_opf &= ~REQ_PREFLUSH;
 		mddev->pers->make_request(mddev, bio);
 	}