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/dm-snap.c b/drivers/md/dm-snap.c
index ce2a910..c65feea 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1680,7 +1680,7 @@
 
 	init_tracked_chunk(bio);
 
-	if (bio->bi_rw & REQ_PREFLUSH) {
+	if (bio->bi_opf & REQ_PREFLUSH) {
 		bio->bi_bdev = s->cow->bdev;
 		return DM_MAPIO_REMAPPED;
 	}
@@ -1800,7 +1800,7 @@
 
 	init_tracked_chunk(bio);
 
-	if (bio->bi_rw & REQ_PREFLUSH) {
+	if (bio->bi_opf & REQ_PREFLUSH) {
 		if (!dm_bio_get_target_bio_nr(bio))
 			bio->bi_bdev = s->origin->bdev;
 		else
@@ -2286,7 +2286,7 @@
 
 	bio->bi_bdev = o->dev->bdev;
 
-	if (unlikely(bio->bi_rw & REQ_PREFLUSH))
+	if (unlikely(bio->bi_opf & REQ_PREFLUSH))
 		return DM_MAPIO_REMAPPED;
 
 	if (bio_data_dir(bio) != WRITE)