tcp: in tcp_close() call tcp_timer_stop() after pcb has been dropped

[glebius]

Summary

The tcp_timer_stop() is designed to be called when the pcb belongs solely
to the calling thread and can't be looked up via in_pcblookup(). The
function may release lock which creates a race if used on a pcb that can
be found by an other thread.

[tuexen]

If the summary is true, wouldn't it be good to KASSERT that (inp->inp_flags & INP_DROPPED) != 0 after we have removed the two calls of tcp_timer_stop() from the RACK code? Willing to work on it...

[glebius]

This doesn't fix anything.