crypto: algif_aead - Fix bogus request dereference in completion function

The algif_aead completion function tries to deduce the aead_request
from the crypto_async_request argument.  This is broken because
the API does not guarantee that the same request will be pased to
the completion function.  Only the value of req->data can be used
in the completion function.

This patch fixes it by storing a pointer to sk in areq and using
that instead of passing in sk through req->data.

Fixes: 83094e5e ("crypto: af_alg - add async support to...")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>