[IPV6] SIT: Sparse: Use NULL pointer instead of 0.

| net/ipv6/sit.c:382:42: warning: Using plain integer as NULL pointer

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index cc16fe0..91e46fb 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -379,7 +379,7 @@
 		dev_put(dev);
 	} else {
 		ipip6_tunnel_unlink(netdev_priv(dev));
-		ipip6_tunnel_del_prl(netdev_priv(dev), 0);
+		ipip6_tunnel_del_prl(netdev_priv(dev), NULL);
 		dev_put(dev);
 	}
 }