tty: Make get_current_tty use a kref

We now return a kref covered tty reference. That ensures the tty structure
doesn't go away when you have a return from get_current_tty. This is not
enough to protect you from most of the resources being freed behind your
back - yet.

[Updated to include fixes for SELinux problems found by Andrew Morton and
 an s390 leak found while debugging the former]

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/s390/char/fs3270.c

@@ -430,11 +430,12 @@ fs3270_open(struct inode *inode, struct file *filp)
 		mutex_lock(&tty_mutex);
 		tty = get_current_tty();
 		if (!tty || tty->driver->major != IBM_TTY3270_MAJOR) {
-			mutex_unlock(&tty_mutex);
+			tty_kref_put(tty);
 			rc = -ENODEV;
 			goto out;
 		}
 		minor = tty->index + RAW3270_FIRSTMINOR;
+		tty_kref_put(tty);
 		mutex_unlock(&tty_mutex);
 	}
 	/* Check if some other program is already using fullscreen mode. */