When a SYN-ACK is received in SYN-SENT state, RFC 793 requires the
validation of SEG.ACK as the first step. If the ACK is not acceptable,
a RST segment should be sent and the segment should be dropped.
Up to now, the segment was partially processed.
This patch moves the check for the SEG.ACK validation up to the front
as required.

Reviewed by: hiren, gnn
Differential Revision: https://reviews.freebsd.org/D10424