devcgroup: make a helper to convert cgroup_subsys_state to devs_cgroup

This is just picking the container_of out of cgroup_to_devcgroup into a
separate function.

This new css_to_devcgroup will be used in the 2nd patch.
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Acked-by: NSerge Hallyn <serue@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: James Morris <jmorris@namei.org>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

security/device_cgroup.c

@@ -49,10 +49,14 @@ struct dev_cgroup {
 	spinlock_t lock;
 };
 
+static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
+{
+	return container_of(s, struct dev_cgroup, css);
+}
+
 static inline struct dev_cgroup *cgroup_to_devcgroup(struct cgroup *cgroup)
 {
-	return container_of(cgroup_subsys_state(cgroup, devices_subsys_id),
-			    struct dev_cgroup, css);
+	return css_to_devcgroup(cgroup_subsys_state(cgroup, devices_subsys_id));
 }
 
 struct cgroup_subsys devices_subsys;