ubifs: Add separate functions to init/crc a node

When adding authentication support we will embed a HMAC into some
nodes. To prepare these nodes we have to first initialize the nodes,
then add a HMAC and finally add a CRC. To accomplish this add separate
ubifs_init_node/ubifs_crc_node functions.

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