rack: unbreak TCP fast open for the client side
Allow sending user data on the SYN segment.
Reviewed by: rrs
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D29082
Sponsored by: Netflix, Inc.
(cherry picked from commit 705d06b289e9821439b7b694d766cad75bc064e5)
(cherry picked from commit 873f8376015c12febec3619976a8c30fbead4ad6)