block: manipulate bio->bi_flags through helpers

Some places use helpers now, others don't. We only have the 'is set'
helper, add helpers for setting and clearing flags too.

It was a bit of a mess of atomic vs non-atomic access. With
BIO_UPTODATE gone, we don't have any risk of concurrent access to the
flags. So relax the restriction and don't make any of them atomic. The
flags that do have serialization issues (reffed and chained), we
already handle those separately.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/block/bounce.c b/block/bounce.c
index f4db245..2c310ea 100644
--- a/block/bounce.c
+++ b/block/bounce.c
@@ -186,7 +186,7 @@
 	if (!bdi_cap_stable_pages_required(&q->backing_dev_info))
 		return 0;
 
-	return test_bit(BIO_SNAP_STABLE, &bio->bi_flags);
+	return bio_flagged(bio, BIO_SNAP_STABLE);
 }
 #else
 static int must_snapshot_stable_pages(struct request_queue *q, struct bio *bio)