ubifs: Fix memory leak in __ubifs_node_verify_hmac error path

In __ubifs_node_verify_hmac(), 'hmac' is allocated through kmalloc().
However, it is not deallocated in the following execution if
ubifs_node_calc_hmac() fails, leading to a memory leak bug. To fix this
issue, free 'hmac' before returning the error.

Fixes: 49525e5e

 ("ubifs: Add helper functions for authentication support")
Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Richard Weinberger <richard@nod.at>