TCP LRO: convert TCP header fields to host byte order earlier

This is a preparation for adding dtrace hooks in a follow-up commit,
which are missing in the code path, where packets are directly queued
to the tcpcb. The dtrace hooks expect the fields to be in host byte
order. This only applies when TCP HPTS is used.
No functional change intended.

Reviewed by: rscheff
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D43594