tcp: no data on SYN unless doing TFO

[tuexen]

Summary

Ensure that there is no data on SYN segments unless doing TFO. This check is already in RACK and BBR.

Test Plan

Ensure the following script passes:

+0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0.000 sendto(3, ..., 100, 0, ..., ...) = 100
+0.000 > S    0:0(0)             win 65535 <mss 1460,nop,wscale 6, sackOK, TS val 1000 ecr 0>
+0.050 < S.   0:0(0)     ack   1 win 65535 <mss 1460>
+0.000 > P.   1:101(100) ack   1 win 65535
+0.050 <  .   1:1(0)     ack 101 win 65535
+0.000 close(3) = 0
+0.000 > F. 101:101(0)   ack   1 win 65535
+0.050 < F.   1:1(0)     ack 102 win 65535
+0.000 >  . 102:102(0)   ack   2 win 65535

[tuexen]

Rebase to main.