tcp: Do not call IPv4 specific func in tcp_check_req

Calling IPv4 specific inet_csk_route_req in tcp_check_req
is a bad idea and crashes machine on IPv6 connections, as reported
by Valdis Kletnieks

Also, all we are really interested in is the timestamp
option in the header, so calling tcp_parse_options()
with the "estab" set to false flag is an overkill as
it tries to parse half a dozen other TCP options.

We know whether timestamp should be enabled or not
using data from request_sock.

Signed-off-by: Gilad Ben-Yossef <gilad@codefidence.com>
Tested-by:  <Valdis.Kletnieks@vt.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>