[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/zone.c b/net/tipc/zone.c
index 7c11f7f..2803e1b 100644
--- a/net/tipc/zone.c
+++ b/net/tipc/zone.c
@@ -44,11 +44,11 @@
 
 struct _zone *tipc_zone_create(u32 addr)
 {
-	struct _zone *z_ptr = 0;
+	struct _zone *z_ptr = NULL;
 	u32 z_num;
 
 	if (!tipc_addr_domain_valid(addr))
-		return 0;
+		return NULL;
 
 	z_ptr = (struct _zone *)kmalloc(sizeof(*z_ptr), GFP_ATOMIC);
 	if (z_ptr != NULL) {
@@ -114,10 +114,10 @@
 	u32 c_num;
 
 	if (!z_ptr)
-		return 0;
+		return NULL;
 	c_ptr = z_ptr->clusters[tipc_cluster(addr)];
 	if (!c_ptr)
-		return 0;
+		return NULL;
 	n_ptr = tipc_cltr_select_node(c_ptr, ref);
 	if (n_ptr)
 		return n_ptr;
@@ -126,12 +126,12 @@
 	for (c_num = 1; c_num <= tipc_max_clusters; c_num++) {
 		c_ptr = z_ptr->clusters[c_num];
 		if (!c_ptr)
-			return 0;
+			return NULL;
 		n_ptr = tipc_cltr_select_node(c_ptr, ref);
 		if (n_ptr)
 			return n_ptr;
 	}
-	return 0;
+	return NULL;
 }
 
 u32 tipc_zone_select_router(struct _zone *z_ptr, u32 addr, u32 ref)