When retransmitting SYN in TCPS_SYN_SENT state use TCPTV_RTOBASE, the default retransmit timeout, as base to calculate the backoff time until next try instead of the TCP_REXMTVAL() macro which only works correctly when we already have measured an actual RTT+RTTVAR.
When retransmitting SYN in TCPS_SYN_SENT state use TCPTV_RTOBASE,
the default retransmit timeout, as base to calculate the backoff
time until next try instead of the TCP_REXMTVAL() macro which only
works correctly when we already have measured an actual RTT+RTTVAR.

Before it would cause the first retransmit at RTOBASE, the next
four at the same time (!) about 200ms later, and then another one
again RTOBASE later.

MFC after:	2 weeks

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