cgroup: introduce CSS_RELEASED and reduce css iteration fallback window

css iterations allow the caller to drop RCU read lock.  As long as the
caller keeps the current position accessible, it can simply re-grab
RCU read lock later and continue iteration.  This is achieved by using
CGRP_DEAD to detect whether the current positions next pointer is safe
to dereference and if not re-iterate from the beginning to the next
position using ->serial_nr.

CGRP_DEAD is used as the marker to invalidate the next pointer and the
only requirement is that the marker is set before the next sibling
starts its RCU grace period.  Because CGRP_DEAD is set at the end of
cgroup_destroy_locked() but the cgroup is unlinked when the reference
count reaches zero, we currently have a rather large window where this
fallback re-iteration logic can be triggered.

This patch introduces CSS_RELEASED which is set when a css is unlinked
from its sibling list.  This still keeps the re-iteration logic
working while drastically reducing the window of its activation.
While at it, rewrite the comment in css_next_child() to reflect the
new flag and better explain the synchronization.

This will also enable iterating csses directly instead of through
cgroups.

v2: CSS_RELEASED now assigned to 1 << 2 as 1 << 0 is used by
    CSS_NO_REF.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>