bcache: Fix sysfs splat on shutdown with flash only devs

Whoops.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
index 4b67244..194d437 100644
--- a/drivers/md/bcache/sysfs.c
+++ b/drivers/md/bcache/sysfs.c
@@ -370,7 +370,7 @@
 	}
 
 	if (attr == &sysfs_unregister) {
-		atomic_set(&d->detaching, 1);
+		set_bit(BCACHE_DEV_DETACHING, &d->flags);
 		bcache_device_stop(d);
 	}