The tcp syncache code was leaving the IPv6 flowlabel uninitialised

The tcp syncache code was leaving the IPv6 flowlabel uninitialised
for the SYN|ACK packet and then letting in6_pcbconnect set the
flowlabel later. Arange for the syncache/syncookie code to set and
recall the flow label so that the flowlabel used for the SYN|ACK
is consistent. This is done by using some of the cookie (when tcp
cookies are enabeled) and by stashing the flowlabel in syncache.

Tested and Discovered by:	Orla McGann <orly@cnri.dit.ie>
Approved by:			ume, silby
MFC after:			1 month