tcp: stop doing superfluous work after sending RST

[rscheff]

Summary

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.

Provided by: glebius

[glebius]

Well, originally my diff. No objection for pushing it in together with your changes. I consider you reviewed it :) Thanks!