tcp: Prepare PRR to work with NewReno LossRecovery

Add proper PRR vnet declarations for consistency.
Also add pointer to tcpopt struct to tcp_do_prr_ack, in preparation
for it to deal with non-SACK window reduction (after loss).

No functional change.

MFC after: 2 weeks
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29440