blkcg: export __blkg_prfill_rwstat()

Hierarchical stats for cfq-iosched will need __blkg_prfill_rwstat().
Export it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 4d625d2..3aec4cd 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -552,6 +552,7 @@
 	seq_printf(sf, "%s Total %llu\n", dname, (unsigned long long)v);
 	return v;
 }
+EXPORT_SYMBOL_GPL(__blkg_prfill_rwstat);
 
 /**
  * blkg_prfill_stat - prfill callback for blkg_stat