ubifs: Set s_uuid in super block to support ima/evm uuid options

This is required to provide uuid based integrity functionality for:
ima_policy (fsuuid option) and the 'evmctl' command ('--uuid' option).

Co-developed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Co-developed-by: Juergen Borleis <jbe@pengutronix.de>
Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>