UPSTREAM: cgroup: Iterate tasks that did not finish do_exit()

PF_EXITING is set earlier than actual removal from css_set when a task
is exitting. This can confuse cgroup.procs readers who see no PF_EXITING
tasks, however, rmdir is checking against css_set membership so it can
transitionally fail with EBUSY.

Fix this by listing tasks that weren't unlinked from css_set active
lists.
It may happen that other users of the task iterator (without
CSS_TASK_ITER_PROCS) spot a PF_EXITING task before cgroup_exit(). This
is equal to the state before commit c03cd773 ("cgroup: Include dying
leaders with live threads in PROCS iterations") but it may be reviewed
later.

Reported-by: Suren Baghdasaryan <surenb@google.com>
Fixes: c03cd773 ("cgroup: Include dying leaders with live threads in PROCS iterations")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
(cherry picked from commit 9c974c77246460fa6a92c18554c3311c8c83c160)
Bug: 141213848
Bug: 146758430
Test: test_cgcore_destroy from linux-kselftest
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Iac57661b931129ed1e44b89675f8115bb89084ff