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-log-writes.c b/drivers/md/dm-log-writes.c
index b5dbf7a..4ab6803 100644
--- a/drivers/md/dm-log-writes.c
+++ b/drivers/md/dm-log-writes.c
@@ -555,8 +555,8 @@
 	struct bio_vec bv;
 	size_t alloc_size;
 	int i = 0;
-	bool flush_bio = (bio->bi_rw & REQ_PREFLUSH);
-	bool fua_bio = (bio->bi_rw & REQ_FUA);
+	bool flush_bio = (bio->bi_opf & REQ_PREFLUSH);
+	bool fua_bio = (bio->bi_opf & REQ_FUA);
 	bool discard_bio = (bio_op(bio) == REQ_OP_DISCARD);
 
 	pb->block = NULL;