crypto: skcipher - add the ability to abort a skcipher walk

After starting a skcipher walk, the only way to ensure that all
resources it has tied up are released is to complete it. In some
cases, it will be useful to be able to abort a walk cleanly after
it has started, so add this ability to the skcipher walk API.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>