tcp: Use defaults when no route options are available

Trying to parse the option of a SYN packet that we have
no route entry for should just use global wide defaults
for route entry options.

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