tcp: move cc_post_recovery past snd_una update

The RFC6675 pipe calculation (sack.revised, enabled
by default since D28702), uses outdated information,
while the previous default calculated it correctly
with up-to-date information from the incoming ACK.

This difference can become as large as the receive
window (not the congestion window previously),
potentially triggering a massive burst of new packets.

MFC after: 1 week
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43520