block: rename bio bi_rw to bi_opf

Since commit 63a4cc2486, 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>

drivers/md/dm-zero.c

@@ -37,7 +37,7 @@ static int zero_map(struct dm_target *ti, struct bio *bio)
 {
 	switch (bio_op(bio)) {
 	case REQ_OP_READ:
-		if (bio->bi_rw & REQ_RAHEAD) {
+		if (bio->bi_opf & REQ_RAHEAD) {
 			/* readahead of null bytes only wastes buffer cache */
 			return -EIO;
 		}