ubifs: Fix memory leak in read_znode() error path

In read_znode(), the indexing node 'idx' is allocated by kmalloc().
However, it is not deallocated in the following execution if
ubifs_node_check_hash() fails, leading to a memory leak bug. To fix this
issue, free 'idx' before returning the error.

Fixes: 16a26b20

 ("ubifs: authentication: Add hashes to index nodes")
Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Richard Weinberger <richard@nod.at>