cgroup/pids: Remove superfluous zeroing

Atomic counters are in kzalloc'd struct. They are zeroed already and
atomic64_t does not need special initialization
(cf kernel/trace/trace_clock.c:trace_counter).

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>