udp6: Use Segment Routing Header for dest address if present

When finding the socket to report an error on, if the invoking packet
is using Segment Routing, the IPv6 destination address is that of an
intermediate router, not the end destination. Extract the ultimate
destination address from the segment address.

This change allows traceroute to function in the presence of Segment
Routing.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>