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

kernel/pid_namespace.c

@@ -52,7 +52,7 @@ static struct kmem_cache *create_pid_cachep(unsigned int level)
 	/* Name collision forces to do allocation under mutex. */
 	if (!*pkc)
 		*pkc = kmem_cache_create(name, len, 0,
-					 SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT, 0);
+					 SLAB_HWCACHE_ALIGN | SLAB_ACCOUNT, NULL);
 	mutex_unlock(&pid_caches_mutex);
 	/* current can fail, but someone else can succeed. */
 	return READ_ONCE(*pkc);