md/raid0: attach correct cgroup info in bio

The discard bio doesn't attach the original bio cgroup info. Normal bio
is cloned, so is fine.

Signed-off-by: Shaohua Li <shli@fb.com>
diff --git a/block/bio.c b/block/bio.c
index 9a63597..51189a1 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2085,7 +2085,7 @@
 	if (src->bi_css)
 		WARN_ON(bio_associate_blkcg(dst, src->bi_css));
 }
-
+EXPORT_SYMBOL_GPL(bio_clone_blkcg_association);
 #endif /* CONFIG_BLK_CGROUP */
 
 static void __init biovec_init_slabs(void)