tty: Update code comment in __proc_set_tty()

The session and foreground process group pid references will be
non-NULL if tiocsctty() is stealing the controlling tty from another
session (ie., arg == 1 in tiocsctty()).
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Reviewed-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -516,8 +516,11 @@ static void __proc_set_tty(struct tty_struct *tty)
 {
 	unsigned long flags;
 
-	/* We should not have a session or pgrp to put here but.... */
 	spin_lock_irqsave(&tty->ctrl_lock, flags);
+	/*
+	 * The session and fg pgrp references will be non-NULL if
+	 * tiocsctty() is stealing the controlling tty
+	 */
 	put_pid(tty->session);
 	put_pid(tty->pgrp);
 	tty->pgrp = get_pid(task_pgrp(current));