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

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
---
 kernel/pid_namespace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index a46a372..f4f8cb0 100644
--- a/kernel/pid_namespace.c
+++ b/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);
-- 
2.7.4