ubifs: Authenticate replayed journal

Make sure that during replay all buds can be authenticated. To do
this we calculate the hash chain until we find an authentication
node and check the HMAC in that node against the current status
of the hash chain.

After a power cut it can happen that some nodes have been written, but
not yet the authentication node for them. These nodes have to be
discarded during replay.

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