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/block/blk-merge.c b/block/blk-merge.c
index 41cbd48..3eec75a 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -186,7 +186,7 @@
 
 	if (split) {
 		/* there isn't chance to merge the splitted bio */
-		split->bi_rw |= REQ_NOMERGE;
+		split->bi_opf |= REQ_NOMERGE;
 
 		bio_chain(split, *bio);
 		trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
@@ -616,9 +616,9 @@
 	 * Distributes the attributs to each bio.
 	 */
 	for (bio = rq->bio; bio; bio = bio->bi_next) {
-		WARN_ON_ONCE((bio->bi_rw & REQ_FAILFAST_MASK) &&
-			     (bio->bi_rw & REQ_FAILFAST_MASK) != ff);
-		bio->bi_rw |= ff;
+		WARN_ON_ONCE((bio->bi_opf & REQ_FAILFAST_MASK) &&
+			     (bio->bi_opf & REQ_FAILFAST_MASK) != ff);
+		bio->bi_opf |= ff;
 	}
 	rq->cmd_flags |= REQ_MIXED_MERGE;
 }