bcache: use op_is_write instead of checking for REQ_WRITE

We currently set REQ_WRITE/WRITE for all non READ IOs
like discard, flush, writesame, etc. In the next patches where we
no longer set up the op as a bitmap, we will not be able to
detect a operation direction like writesame by testing if REQ_WRITE is
set.

This has bcache use the op_is_write helper which will do the right
thing.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
index 86a0bb8..fd885cc 100644
--- a/drivers/md/bcache/io.c
+++ b/drivers/md/bcache/io.c
@@ -111,7 +111,7 @@
 	struct bbio *b = container_of(bio, struct bbio, bio);
 	struct cache *ca = PTR_CACHE(c, &b->key, 0);
 
-	unsigned threshold = bio->bi_rw & REQ_WRITE
+	unsigned threshold = op_is_write(bio_op(bio))
 		? c->congested_write_threshold_us
 		: c->congested_read_threshold_us;