tcp: reduce the size of t_rttupdated in tcpcb

During tcp session start, various mechanisms need to
track a few initial RTTs before becoming active.
Prevent overflows of the corresponding tracking counter
and reduce the size of tcpcb simultaneously.

Reviewed By: transport, tuexen, guest-ccui
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D21117