ubifs: add check for crypto_shash_tfm_digest

Add check for the return value of crypto_shash_tfm_digest() and return
the error if it fails in order to catch the error.

Fixes: 817aa094 ("ubifs: support offline signed images")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>