Fix an issue when TSO and Rack play together. Basically

an retransmission of the initial SYN (with data) would
cause us to strip the SYN and decrement/increase offset/len
which then caused us a -1 offset and a panic.

Reported by:	Larry Rosenman
(Michael Tuexen helped me debug this at the IETF)

sys/netinet/tcp_stacks/rack.c

@@ -7405,9 +7405,6 @@ rack_output(struct tcpcb *tp)
 		    (tp->t_state == TCPS_SYN_RECEIVED))
 			flags &= ~TH_SYN;
 #endif
-		sb_offset--, len++;
-		if (sbavail(sb) == 0)
-			len = 0;
 	}
 	/*
 	 * Be careful not to send data and/or FIN on SYN segments. This