cgroups: subsystem module unloading

cgroups: subsystem module unloading

Provides support for unloading modular subsystems.

This patch adds a new function cgroup_unload_subsys which is to be used
for removing a loaded subsystem during module deletion.  Reference
counting of the subsystems' modules is moved from once (at load time) to
once per attached hierarchy (in parse_cgroupfs_options and
rebind_subsystems) (i.e., 0 or 1).

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: Linus Torvalds <[email protected]>