ubifs: Refactor create_default_filesystem()

create_default_filesystem() allocates memory for a node, writes that
node and frees the memory directly afterwards. With this patch we
allocate memory for all nodes at the beginning of the function and
free the memory at the end. This makes it easier to implement
authentication support since with authentication support we'll need
the contents of some nodes when creating other nodes.

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