ipv6: ip6_skb_dst_mtu() cleanups

Use const attribute where we can, and cache skb_dst()
Signed-off-by: NEric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211119022355.2985984-1-eric.dumazet@gmail.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

include/net/ip6_route.h

@@ -263,19 +263,19 @@ static inline bool ipv6_anycast_destination(const struct dst_entry *dst,
 int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 		 int (*output)(struct net *, struct sock *, struct sk_buff *));
 
-static inline unsigned int ip6_skb_dst_mtu(struct sk_buff *skb)
+static inline unsigned int ip6_skb_dst_mtu(const struct sk_buff *skb)
 {
-	unsigned int mtu;
-
-	struct ipv6_pinfo *np = skb->sk && !dev_recursion_level() ?
+	const struct ipv6_pinfo *np = skb->sk && !dev_recursion_level() ?
 				inet6_sk(skb->sk) : NULL;
+	const struct dst_entry *dst = skb_dst(skb);
+	unsigned int mtu;
 
 	if (np && np->pmtudisc >= IPV6_PMTUDISC_PROBE) {
-		mtu = READ_ONCE(skb_dst(skb)->dev->mtu);
-		mtu -= lwtunnel_headroom(skb_dst(skb)->lwtstate, mtu);
-	} else
-		mtu = dst_mtu(skb_dst(skb));
-
+		mtu = READ_ONCE(dst->dev->mtu);
+		mtu -= lwtunnel_headroom(dst->lwtstate, mtu);
+	} else {
+		mtu = dst_mtu(dst);
+	}
 	return mtu;
 }