kernel: pid_namespace: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
kernel/pid_namespace.c:55:77: warning: Using plain integer as NULL pointer

Link: https://lkml.kernel.org/r/1647944288-2806-1-git-send-email-baihaowen@meizu.com


Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>