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/block/umem.c b/drivers/block/umem.c
index d0a3e6d..be90e15 100644
--- a/drivers/block/umem.c
+++ b/drivers/block/umem.c
@@ -535,7 +535,7 @@
 	*card->biotail = bio;
 	bio->bi_next = NULL;
 	card->biotail = &bio->bi_next;
-	if (bio->bi_rw & REQ_SYNC || !mm_check_plugged(card))
+	if (bio->bi_opf & REQ_SYNC || !mm_check_plugged(card))
 		activate(card);
 	spin_unlock_irq(&card->lock);