ubifs: Store read superblock node

The superblock node is read/modified/written several times throughout
the UBIFS code. Instead of reading it from the device each time just
keep a copy in memory and write back the modified copy when necessary.
This patch helps for authentication support, here we not only have to
read the superblock node, but also have to authenticate it, which
is easier if we do it once during initialization.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>