The `events_limit` variable needs to be formatted with %lld and not %ld.
This fixes the following warning discovered by kbuild test robot:

   kernel/cgroup_pids.c: In function 'pids_events_show':
   kernel/cgroup_pids.c:313:24: warning: format '%ld' expects argument of type
   'long int', but argument 3 has type 'long long int' [-Wformat=]
        seq_printf(sf, "max %ld\n", atomic64_read(&pids->events_limit));
                                   ^

tj: Added explicit (s64) cast as atomic64 switches between long long
    and long depending on 32 or 64.

Signed-off-by: Kenny Yu <kennyyu@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>