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/bio.c b/block/bio.c
index 3f76a38..f394775 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -580,7 +580,7 @@
 	 */
 	bio->bi_bdev = bio_src->bi_bdev;
 	bio_set_flag(bio, BIO_CLONED);
-	bio->bi_rw = bio_src->bi_rw;
+	bio->bi_opf = bio_src->bi_opf;
 	bio->bi_iter = bio_src->bi_iter;
 	bio->bi_io_vec = bio_src->bi_io_vec;
 
@@ -663,7 +663,7 @@
 	if (!bio)
 		return NULL;
 	bio->bi_bdev		= bio_src->bi_bdev;
-	bio->bi_rw		= bio_src->bi_rw;
+	bio->bi_opf		= bio_src->bi_opf;
 	bio->bi_iter.bi_sector	= bio_src->bi_iter.bi_sector;
 	bio->bi_iter.bi_size	= bio_src->bi_iter.bi_size;
 
@@ -873,7 +873,7 @@
 	init_completion(&ret.event);
 	bio->bi_private = &ret;
 	bio->bi_end_io = submit_bio_wait_endio;
-	bio->bi_rw |= REQ_SYNC;
+	bio->bi_opf |= REQ_SYNC;
 	submit_bio(bio);
 	wait_for_completion_io(&ret.event);