UBIFS: fix error path in create_default_filesystem()

In the end of 'create_default_filesystem()' we need to check
the return value of 'ubifs_write_node()' to ensure that we have
successfully written the 'cs_node'.

Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>