ecryptfs: remove unnessesary null check in ecryptfs_keyring_auth_tok_for_sig

request_key and ecryptfs_get_encrypted_key never
return a NULL pointer, so no need do a null check.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>