cifsd: simplify error handling in ksmbd_gen_preauth_integrity_hash()

Simplify error handling in ksmbd_gen_preauth_integrity_hash().

Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>