bcache: Fix a null ptr deref regression

Commit c0f04d88 ("bcache: Fix flushes in writeback mode") was fixing
a reported data corruption bug, but it seems some last minute
refactoring or rebasing introduced a null pointer deref.
Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: linux-stable <stable@vger.kernel.org> # >= v3.10
Reported-by: Gabriel de Perthuis <g2p.code@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/bcache/request.c

@@ -996,6 +996,7 @@ static void request_write(struct cached_dev *dc, struct search *s)
 		closure_bio_submit(bio, cl, s->d);
 	} else {
 		bch_writeback_add(dc);
+		s->op.cache_bio = bio;
 
 		if (bio->bi_rw & REQ_FLUSH) {
 			/* Also need to send a flush to the backing device */
@@ -1008,8 +1009,6 @@ static void request_write(struct cached_dev *dc, struct search *s)
 			flush->bi_private = cl;
 
 			closure_bio_submit(flush, cl, s->d);
-		} else {
-			s->op.cache_bio = bio;
 		}
 	}
 out: