devcgroup: make a helper to convert cgroup_subsys_state to devs_cgroup

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: Pavel Emelyanov <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: James Morris <[email protected]>
Cc: Chris Wright <[email protected]>
Cc: Stephen Smalley <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>