bcache: Fix a lockdep splat

bch_keybuf_del() takes a spinlock that can't be taken in interrupt context -
whoops. Fortunately, this code isn't enabled by default (you have to toggle a
sysfs thing).
Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/movinggc.c

@@ -113,7 +113,7 @@ static void write_moving(struct closure *cl)
 		bch_insert_data(&s->op.cl);
 	}
 
-	continue_at(cl, write_moving_finish, NULL);
+	continue_at(cl, write_moving_finish, bch_gc_wq);
 }
 
 static void read_moving_submit(struct closure *cl)