tcp rack: improve initialisation of retransmit timeout
When the TCP is in the front states, don't take the slop variable
into account. This improves consistency with the base stack.
Reviewed by: rrs@
Differential Revision: https://reviews.freebsd.org/D30230
Sponsored by: Netflix, Inc.
(cherry picked from commit 251842c63927fc4af63bdc61989bbfbf3823c679)