dm crypt: fix memory leak in crypt_ctr_cipher_old()

Fix memory leak of cipher_api.

Fixes: 33d2f09f

 (dm crypt: introduce new format of cipher with "capi:" prefix)
Cc: stable@vger.kernel.org # 4.12+
Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>