[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/config.c b/net/tipc/config.c
index 3c8e674..48b5de2 100644
--- a/net/tipc/config.c
+++ b/net/tipc/config.c
@@ -683,11 +683,11 @@
 	memset(&mng, 0, sizeof(mng));
 	INIT_LIST_HEAD(&mng.link_subscribers);
 
-	res = tipc_attach(&mng.user_ref, 0, 0);
+	res = tipc_attach(&mng.user_ref, NULL, NULL);
 	if (res)
 		goto failed;
 
-	res = tipc_createport(mng.user_ref, 0, TIPC_CRITICAL_IMPORTANCE,
+	res = tipc_createport(mng.user_ref, NULL, TIPC_CRITICAL_IMPORTANCE,
 			      NULL, NULL, NULL,
 			      NULL, cfg_named_msg_event, NULL,
 			      NULL, &mng.port_ref);