cgroup: add/update accessors which obtain subsys specific data from css

css (cgroup_subsys_state) is usually embedded in a subsys specific
data structure.  Subsystems either use container_of() directly to cast
from css to such data structure or has an accessor function wrapping
such cast.  As cgroup as whole is moving towards using css as the main
interface handle, add and update such accessors to ease dealing with
css's.

All accessors explicitly handle NULL input and return NULL in those
cases.  While this looks like an extra branch in the code, as all
controllers specific data structures have css as the first field, the
casting doesn't involve any offsetting and the compiler can trivially
optimize out the branch.

* blkio, freezer, cpuset, cpu, cpuacct and net_cls didn't have such
  accessor.  Added.

* memory, hugetlb and devices already had one but didn't explicitly
  handle NULL input.  Updated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/sched/cpuacct.c

@@ -33,18 +33,21 @@ struct cpuacct {
 	struct kernel_cpustat __percpu *cpustat;
 };
 
+static inline struct cpuacct *css_ca(struct cgroup_subsys_state *css)
+{
+	return css ? container_of(css, struct cpuacct, css) : NULL;
+}
+
 /* return cpu accounting group corresponding to this container */
 static inline struct cpuacct *cgroup_ca(struct cgroup *cgrp)
 {
-	return container_of(cgroup_css(cgrp, cpuacct_subsys_id),
-			    struct cpuacct, css);
+	return css_ca(cgroup_css(cgrp, cpuacct_subsys_id));
 }
 
 /* return cpu accounting group to which this task belongs */
 static inline struct cpuacct *task_ca(struct task_struct *tsk)
 {
-	return container_of(task_css(tsk, cpuacct_subsys_id),
-			    struct cpuacct, css);
+	return css_ca(task_css(tsk, cpuacct_subsys_id));
 }
 
 static inline struct cpuacct *__parent_ca(struct cpuacct *ca)