An ldisc reference is insufficient guarantee the foreground process
group is not in the process of being signalled from a hangup.

1) Reads of tty->pgrp must be locked with ctrl_lock
2) The group pid must be referenced for the duration of signalling.
   Because the driver-side is not process-context, a pid reference
   must be acquired.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>