ipv6: add missing ireq_net & ir_cookie initializations

I forgot to update dccp_v6_conn_request() & cookie_v6_check().
They both need to set ireq->ireq_net and ireq->ir_cookie

Lets clear ireq->ir_cookie in inet_reqsk_alloc()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 33cf7c90 ("net: add real socket cookies")
Signed-off-by: David S. Miller <davem@davemloft.net>