If the inpcb is already bound to a local address, there is no reason to
call in6_pcbladdr(). If the inpcb is already bound to a local port, there
is no reason to call in_pcb_lport_dest(). In the opposite case, if the
inpcb is not bound, and we are about to choose a non-conflicting local
addr:port, then there is no reason to call in6_pcblookup_internal().
This change makes in6_pcbconnect() to look much more alike the IPv4
in_pcbconnect(). I tracked this strange logic all the way down to initial
KAME import and failed to find any reasoning for it.