tcp: Tidying up the conditionals for unwinding a spurious RTO
- Use the semantically correct TSTMP_xx macro when comparing timestamps. (No functional change)
- check for bad retransmits only when TSopt is present in ACK (don't assume there will be a valid TSopt in the TCP options struct)
- exclude tsecr == 0, since that most likely indicates an invalid ts echo return (tsecr) value.