[TCP]: SACK enhanced FRTO

Implements the SACK-enhanced FRTO given in RFC4138 using the
variant given in Appendix B.

RFC4138, Appendix B:
  "This means that in order to declare timeout spurious, the TCP
   sender must receive an acknowledgment for non-retransmitted
   segment between SND.UNA and RecoveryPoint in algorithm step 3.
   RecoveryPoint is defined in conservative SACK-recovery
   algorithm [RFC3517]"

The basic version of the FRTO algorithm can still be used also
when SACK is enabled. To enabled SACK-enhanced version, tcp_frto
sysctl is set to 2.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>