tty: fix read of tty->pgrp outside of ctrl_lock

The intention was clearly to use the tty_pgrp local variable rather than
re-read tty->pgrp outside of ctrl_lock, so do that.

This bug was introduced by commit 2812d9e9

 ("tty: Combine
SIGTTOU/SIGTTIN handling").

Signed-off-by: David Emett <dave@sp4m.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>