[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/ref.h b/net/tipc/ref.h
index 4f8f9f40..6d20006 100644
--- a/net/tipc/ref.h
+++ b/net/tipc/ref.h
@@ -92,7 +92,7 @@
 			return r->object;
 		spin_unlock_bh(&r->lock);
 	}
-	return 0;
+	return NULL;
 }
 
 /**
@@ -125,7 +125,7 @@
 		if (likely(r->data.reference == ref))
 			return r->object;
 	}
-	return 0;
+	return NULL;
 }
 
 #endif