Patch from Rik van Riel <riel@conectiva.com.br>

the following patch, against today's BK tree, fixes a small
SMP race in disassociate_ctty.  This function gets called
from do_exit, without the BKL held.

However, it sets the *tty variable before grabbing the bkl,
then makes decisions on what the variable was set to before
the lock was grabbed, despite the fact that another process
could modify its ->tty pointer in this same function.