cgroup: Fix css_task_iter_advance_css_set() cset skip condition

While adding handling for dying task group leaders c03cd773


("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: c03cd773

 ("cgroup: Include dying leaders with live threads in PROCS iterations")
Reported-by:  <syzbot+d4bba5ccd4f9a2a68681@syzkaller.appspotmail.com>