ip_tunnel: replace dst_cache with generic implementation

The current ip_tunnel cache implementation is prone to a race
that will cause the wrong dst to be cached on cuncurrent dst cache
miss and ip tunnel update via netlink.

Replacing with the generic implementation fix the issue.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Suggested-and-acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index 7758247..395d827 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -186,6 +186,7 @@
 
 config NET_IP_TUNNEL
 	tristate
+	select DST_CACHE
 	default n
 
 config NET_IPGRE