tcp: provide correct snd_fack on post_recovery

Ensure that snd_fack holds a valid value when doing
the post_recovery CC processing, for preparation of
the cc_cubic update, so that local pipe calculations
can correctly refer to snd_fack during and after CC events.

Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43957