[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/cluster.h b/net/tipc/cluster.h
index 9963642..1b4cd30 100644
--- a/net/tipc/cluster.h
+++ b/net/tipc/cluster.h
@@ -86,7 +86,7 @@
 
 	if (z_ptr)
 		return z_ptr->clusters[1];
-	return 0;
+	return NULL;
 }
 
 #endif