ip_tunnel: Preserve pointer const in ip_tunnel_info_opts

Change ip_tunnel_info_opts( ) from static function to macro to cast return
value and preserve the const-ness of the pointer.

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/ip_tunnels.h

@@ -67,6 +67,12 @@ struct ip_tunnel_key {
 	GENMASK((sizeof_field(struct ip_tunnel_info,		\
 			      options_len) * BITS_PER_BYTE) - 1, 0)
 
+#define ip_tunnel_info_opts(info)				\
+	_Generic(info,						\
+		 const struct ip_tunnel_info * : ((const void *)((info) + 1)),\
+		 struct ip_tunnel_info * : ((void *)((info) + 1))\
+	)
+
 struct ip_tunnel_info {
 	struct ip_tunnel_key	key;
 #ifdef CONFIG_DST_CACHE
@@ -485,11 +491,6 @@ static inline void iptunnel_xmit_stats(struct net_device *dev, int pkt_len)
 	}
 }
 
-static inline void *ip_tunnel_info_opts(struct ip_tunnel_info *info)
-{
-	return info + 1;
-}
-
 static inline void ip_tunnel_info_opts_get(void *to,
 					   const struct ip_tunnel_info *info)
 {