cgroup: Fix css_task_iter_advance_css_set() cset skip condition

commit c596687a008b579c503afb7a64fcacc7270fae9e upstream.

While adding handling for dying task group leaders c03cd7738a83
("cgroup: Include dying leaders with live threads in PROCS
iterations") added an inverted cset skip condition to
css_task_iter_advance_css_set().  It should skip cset if it's
completely empty but was incorrectly testing for the inverse condition
for the dying_tasks list.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: c03cd7738a83 ("cgroup: Include dying leaders with live threads in PROCS iterations")
Reported-by:  <syzbot+d4bba5ccd4f9a2a68681@syzkaller.appspotmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>