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.

Reviewed By: tuexen, transport
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D34062