ipv6: constify rt6_nexthop()

There is no functional change in this patch, it only prepares the next one.

rt6_nexthop() will be used by ip6_dst_lookup_neigh(), which uses const
variables.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 4790bea..ee7405e 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -262,8 +262,8 @@ static inline bool ip6_sk_ignore_df(const struct sock *sk)
 	       inet6_sk(sk)->pmtudisc == IPV6_PMTUDISC_OMIT;
 }
 
-static inline struct in6_addr *rt6_nexthop(struct rt6_info *rt,
-					   struct in6_addr *daddr)
+static inline const struct in6_addr *rt6_nexthop(const struct rt6_info *rt,
+						 const struct in6_addr *daddr)
 {
 	if (rt->rt6i_flags & RTF_GATEWAY)
 		return &rt->rt6i_gateway;