blkcg: implement task_get_blkcg_css()

Implement a wrapper around task_get_css() to acquire the blkcg css for
a given task.  The wrapper is necessary for cgroup writeback support
as there will be places outside blkcg proper trying to acquire
blkcg_css and blkio_cgrp_id will be undefined when !CONFIG_BLK_CGROUP.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk-cgroup.h

@@ -195,6 +195,12 @@ static inline struct blkcg *bio_blkcg(struct bio *bio)
 	return task_blkcg(current);
 }
 
+static inline struct cgroup_subsys_state *
+task_get_blkcg_css(struct task_struct *task)
+{
+	return task_get_css(task, blkio_cgrp_id);
+}
+
 /**
  * blkcg_parent - get the parent of a blkcg
  * @blkcg: blkcg of interest
@@ -573,6 +579,12 @@ struct blkcg_policy {
 
 #define blkcg_root_css	((struct cgroup_subsys_state *)ERR_PTR(-EINVAL))
 
+static inline struct cgroup_subsys_state *
+task_get_blkcg_css(struct task_struct *task)
+{
+	return NULL;
+}
+
 #ifdef CONFIG_BLOCK
 
 static inline struct blkcg_gq *blkg_lookup(struct blkcg *blkcg, void *key) { return NULL; }