ubifs: Fix memleak in ubifs_init_authentication

When crypto_shash_digestsize() fails, c->hmac_tfm
has not been freed before returning, which leads
to memleak.

Fixes: 49525e5e

 ("ubifs: Add helper functions for authentication support")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>