l2tp: fix locking of 64-bit counters for smp

L2TP uses 64-bit counters but since these are not updated atomically,
we need to make them safe for smp. This patch addresses that.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_core.h b/net/l2tp/l2tp_core.h
index 09e4a38..a8c943b 100644
--- a/net/l2tp/l2tp_core.h
+++ b/net/l2tp/l2tp_core.h
@@ -45,6 +45,7 @@
 	u64			rx_oos_packets;
 	u64			rx_errors;
 	u64			rx_cookie_discards;
+	struct u64_stats_sync	syncp;
 };
 
 struct l2tp_tunnel;