MFC r316065: Enable route and LLE (ndp) caching in TCP/IPv6

tcp_output.c was using a route on the stack for IPv6, which does not
allow route caching or LLE/ndp caching. Switch to using the route
(v6 flavor) in the in_pcb, which was already present, which caches
both L3 and L2 lookups.

Reviewed by: gnn hiren