[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/name_distr.c b/net/tipc/name_distr.c
index 830f909..953307a 100644
--- a/net/tipc/name_distr.c
+++ b/net/tipc/name_distr.c
@@ -168,8 +168,8 @@
 void tipc_named_node_up(unsigned long node)
 {
 	struct publication *publ;
-	struct distr_item *item = 0;
-	struct sk_buff *buf = 0;
+	struct distr_item *item = NULL;
+	struct sk_buff *buf = NULL;
 	u32 left = 0;
 	u32 rest;
 	u32 max_item_buf;
@@ -200,7 +200,7 @@
 			    "<%u.%u.%u>\n", tipc_zone(node), 
 			    tipc_cluster(node), tipc_node(node));
 			tipc_link_send(buf, node, node);
-			buf = 0;
+			buf = NULL;
 		}
 	}
 exit: