Provides support for unloading modular subsystems.

From: Ben Blum <redacted>

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).

It also adds a proper module_delete call in net/sched/cls_cgroup.c.

Signed-off-by: Ben Blum <redacted>

Acked-by: Li Zefan <redacted>

Also please fix comment style and add some blank lines properly.