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/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index bb0addc..51f1255 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6012,7 +6012,7 @@
 				else
 					btrfs_dev_stat_inc(dev,
 						BTRFS_DEV_STAT_READ_ERRS);
-				if ((bio->bi_rw & WRITE_FLUSH) == WRITE_FLUSH)
+				if ((bio->bi_opf & WRITE_FLUSH) == WRITE_FLUSH)
 					btrfs_dev_stat_inc(dev,
 						BTRFS_DEV_STAT_FLUSH_ERRS);
 				btrfs_dev_stat_print_on_error(dev);
@@ -6089,7 +6089,7 @@
 	bio->bi_next = NULL;
 
 	spin_lock(&device->io_lock);
-	if (bio->bi_rw & REQ_SYNC)
+	if (bio->bi_opf & REQ_SYNC)
 		pending_bios = &device->pending_sync_bios;
 	else
 		pending_bios = &device->pending_bios;
@@ -6127,7 +6127,7 @@
 		rcu_read_lock();
 		name = rcu_dereference(dev->name);
 		pr_debug("btrfs_map_bio: rw %d 0x%x, sector=%llu, dev=%lu "
-			 "(%s id %llu), size=%u\n", bio_op(bio), bio->bi_rw,
+			 "(%s id %llu), size=%u\n", bio_op(bio), bio->bi_opf,
 			 (u64)bio->bi_iter.bi_sector, (u_long)dev->bdev->bd_dev,
 			 name->str, dev->devid, bio->bi_iter.bi_size);
 		rcu_read_unlock();