ubifs: Pass ubifs_zbranch to read_znode()

read_znode() takes len, lnum and offs arguments which the caller all
extracts from the same struct ubifs_zbranch *. When adding authentication
support we would have to add a pointer to a hash to the arguments which
is also part of struct ubifs_zbranch. Pass the ubifs_zbranch * instead
so that we do not have to add another argument.

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