Correct a bug introduced with reduced TCP state handling; make
sure that the MAC label on TCP responses during TIMEWAIT is
properly set from either the socket (if available), or the mbuf
that it's responding to.

Unfortunately, this is made somewhat difficult by the TCP code,
as tcp_twstart() calls tcp_twrespond() after discarding the socket
but without a reference to the mbuf that causes the "response".
Passing both the socket and the mbuf works arounds this--eventually
it might be good to make sure the mbuf always gets passed in in
"response" scenarios but working through this provided to
complicate things too much.

Approved by: re (scottl)
Reviewed by: hsu
Obtained from: TrustedBSD Project
Sponsored by: DARPA, Network Associates Laboratories