tcp/sack: Take FIN bit into consideration when setup report SACK block end

tcp/sack: Take FIN bit into consideration when setup report SACK block end

The invalid SACK option observed on the heavily segment reordering network
path (50% segments out of order) contains only one SACK block which tries
to cover the FIN segment but sets the right edge incorrectly, i.e. the right
edge is same as the left edge; mainly because the FIN bit is not counted.