inet: constify ip headers and in6_addr

Add const qualifiers to structs iphdr, ipv6hdr and in6_addr pointers
where possible, to make code intention more obvious.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index e9aee81..f4b7f80 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1507,7 +1507,7 @@
 	return 68;
 }
 
-unsigned short ip_rt_frag_needed(struct net *net, struct iphdr *iph,
+unsigned short ip_rt_frag_needed(struct net *net, const struct iphdr *iph,
 				 unsigned short new_mtu,
 				 struct net_device *dev)
 {