tty: add missing rcu_read_lock for task_pgrp

task_pgrp requires an rcu or tasklist lock to be obtained if the returned pid
is to be dereferenced, which kill_pgrp does. Obtain an RCU lock for the
duration of use.

Signed-off-by: Patrick Donnelly <batrick@batbytes.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc drivers/tty/n_tty.c
Simple merge

diff --cc drivers/tty/tty_io.c
Simple merge