ipv6: mark address parameters of udp_tunnel6_xmit_skb() as const

The function doesn't modify the addresses passed as input, mark them
as 'const' to make that clear.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20230924153014.786962-1-b.galvani@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>