ipv6: remove "proto" argument from udp_tunnel6_dst_lookup()

The function is now UDP-specific, the protocol is always IPPROTO_UDP.

This is similar to what already done for IPv4 in commit 78f3655a
("ipv4: remove "proto" argument from udp_tunnel_dst_lookup()").

Suggested-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>