ipv6: ip6_skb_dst_mtu() cleanups

Use const attribute where we can, and cache skb_dst()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211119022355.2985984-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>