inet: makes syn_ack_timeout mandatory

There are two struct request_sock_ops providers, tcp and dccp.

inet_csk_reqsk_queue_prune() can avoid testing syn_ack_timeout being
NULL if we make it non NULL like syn_ack_timeout

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Cc: dccp@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>