cgroups: net_cls as module

cgroups: net_cls as module

Allows the net_cls cgroup subsystem to be compiled as a module

This patch modifies net/sched/cls_cgroup.c to allow the net_cls subsystem
to be optionally compiled as a module instead of builtin.  The
cgroup_subsys struct is moved around a bit to allow the subsys_id to be
either declared as a compile-time constant by the cgroup_subsys.h include
in cgroup.h, or, if it's a module, initialized within the struct by
cgroup_load_subsys.

Signed-off-by: Ben Blum <[email protected]>
Acked-by: Li Zefan <[email protected]>
Cc: Paul Menage <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: David S. Miller <[email protected]>