ubifs: Add authentication nodes to journal

Nodes that are written to flash can only be authenticated through the
index after the next commit. When a journal replay is necessary the
nodes are not yet referenced by the index and thus can't be
authenticated.

This patch overcomes this situation by creating a hash over all nodes
beginning from the commit start node over the reference node(s) and
the buds themselves. From
time to time we insert authentication nodes. Authentication nodes
contain a HMAC from the current hash state, so that they can be
used to authenticate a journal replay up to the point where the
authentication node is. The hash is continued afterwards
so that theoretically we would only have to check the HMAC of
the last authentication node we find.

Overall we get this picture:

,,,,,,,,
,......,...........................................
,. CS  ,               hash1.----.           hash2.----.
,.  |  ,                    .    |hmac            .    |hmac
,.  v  ,                    .    v                .    v
,.REF#0,-> bud -> bud -> bud.-> auth -> bud -> bud.-> auth ...
,..|...,...........................................
,  |   ,
,  |   ,,,,,,,,,,,,,,,
.  |            hash3,----.
,  |                 ,    |hmac
,  v                 ,    v
, REF#1 -> bud -> bud,-> auth ...
,,,|,,,,,,,,,,,,,,,,,,
   v
  REF#2 -> ...
   |
   V
  ...

Note how hash3 covers CS, REF#0 and REF#1 so that it is not possible to
exchange or skip any reference nodes. Unlike the picture suggests the
auth nodes themselves are not hashed.

With this it is possible for an offline attacker to cut each journal
head or to drop the last reference node(s), but not to skip any journal
heads or to reorder any operations.

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