blk-cgroup: separate out blkg_rwstat under CONFIG_BLK_CGROUP_RWSTAT

blkg_rwstat is now only used by bfq-iosched and blk-throtl when on
cgroup1.  Let's move it into its own files and gate it behind a config
option.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 2676c06..9c82c1f 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -10,6 +10,8 @@
 #include <linux/hrtimer.h>
 #include <linux/blk-cgroup.h>
 
+#include "blk-cgroup-rwstat.h"
+
 #define BFQ_IOPRIO_CLASSES	3
 #define BFQ_CL_IDLE_TIMEOUT	(HZ/5)