MFC r340782:

A TCP stack is required to check SEG.ACK first, when processing a
segment in the SYN-SENT state as stated in Section 3.9 of RFC 793,
page 66. Ensure this is also done by the TCP RACK stack.

Reviewed by: rrs@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D18034