[TIPC]: Fix simple sparse warnings

Tried to run the new tipc stack through sparse.
Following patch fixes all cases where 0 was used
as replacement of NULL.
Use NULL to document this is a pointer and to silence sparse.

This brough sparse warning count down with 127 to 24 warnings.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/user_reg.c b/net/tipc/user_reg.c
index 106200d..3f3f933 100644
--- a/net/tipc/user_reg.c
+++ b/net/tipc/user_reg.c
@@ -65,7 +65,7 @@
 #define MAX_USERID 64
 #define USER_LIST_SIZE ((MAX_USERID + 1) * sizeof(struct tipc_user))
 
-static struct tipc_user *users = 0;
+static struct tipc_user *users = NULL;
 static u32 next_free_user = MAX_USERID + 1;
 static spinlock_t reg_lock = SPIN_LOCK_UNLOCKED;
 
@@ -149,7 +149,7 @@
 			reg_callback(&users[id]);
 	}
 	kfree(users);
-	users = 0;
+	users = NULL;
 }
 
 /**