USB: serial: console: fix uninitialised spinlock

Since commit 4a510969374a ("tty: Make tty_files_lock per-tty") a new
tty_struct spin lock is taken in the tty release path, but the
USB-serial-console hack was never updated hence leaving the lock of its
"fake" tty uninitialised. This was eventually detected by lockdep.

Make sure to initialise the new lock also for the fake tty to address
this regression.

Yes, this code is a mess, but cleaning it up is left for another day.

Fixes: 4a510969374a ("tty: Make tty_files_lock per-tty")
Cc: stable <stable@vger.kernel.org>	# 4.6
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/console.c

@@ -143,6 +143,7 @@ static int usb_console_setup(struct console *co, char *options)
 			tty->driver = usb_serial_tty_driver;
 			tty->index = co->index;
 			init_ldsem(&tty->ldisc_sem);
+			spin_lock_init(&tty->files_lock);
 			INIT_LIST_HEAD(&tty->tty_files);
 			kref_get(&tty->driver->kref);
 			__module_get(tty->driver->owner);