tcp: stop doing superfluous work after sending RST

When sending a RST control segment in tcp_output() it
means we are in TCPS_CLOSED state, called from tcp_drop().
Once the RST is sent, don't call tcp_timer_activate() or
update anything in tcpcb, since that will go away shortly.

PR: 276761
Provided by: glebius
Reviewed By: glebius, tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D43808