blkcg: blkg_rwstat_read() was missing inline

blkg_rwstat_read() in blk-cgroup.h was missing inline modifier causing
compile warning depending on configuration.  Add it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.h

@@ -279,7 +279,7 @@ static inline void blkg_rwstat_add(struct blkg_rwstat *rwstat,
  * This function can be called without synchronization and takes care of
  * u64 atomicity.
  */
-static struct blkg_rwstat blkg_rwstat_read(struct blkg_rwstat *rwstat)
+static inline struct blkg_rwstat blkg_rwstat_read(struct blkg_rwstat *rwstat)
 {
 	unsigned int start;
 	struct blkg_rwstat tmp;