cgroup: make css->refcnt clearing on cgroup removal optional

Currently, cgroup removal tries to drain all css references.  If there
are active css references, the removal logic waits and retries
->pre_detroy() until either all refs drop to zero or removal is
cancelled.

This semantics is unusual and adds non-trivial complexity to cgroup
core and IMHO is fundamentally misguided in that it couples internal
implementation details (references to internal data structure) with
externally visible operation (rmdir).  To userland, this is a behavior
peculiarity which is unnecessary and difficult to expect (css refs is
otherwise invisible from userland), and, to policy implementations,
this is an unnecessary restriction (e.g. blkcg wants to hold css refs
for caching purposes but can't as that becomes visible as rmdir hang).

Unfortunately, memcg currently depends on ->pre_destroy() retrials and
cgroup removal vetoing and can't be immmediately switched to the new
behavior.  This patch introduces the new behavior of not waiting for
css refs to drain and maintains the old behavior for subsystems which
have __DEPRECATED_clear_css_refs set.

Once, memcg is updated, we can drop the code paths for the old
behavior as proposed in the following patch.  Note that the following
patch is incorrect in that dput work item is in cgroup and may lose
some of dputs when multiples css's are released back-to-back, and
__css_put() triggers check_for_release() when refcnt reaches 0 instead
of 1; however, it shows what part can be removed.

  http://thread.gmane.org/gmane.linux.kernel.containers/22559/focus=75251

Note that, in not-too-distant future, cgroup core will start emitting
warning messages for subsys which require the old behavior, so please
get moving.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>