crypto: algif_skcipher - Do not assume that req is unchanged

The async path in algif_skcipher assumes that the crypto completion
function will be called with the original request.  This is not
necessarily the case.  In fact there is no need for this anyway
since we already embed information into the request with struct
skcipher_async_req.

This patch adds a pointer to that struct and then passes it as
the data to the callback function.

Cc: stable@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Tadeusz Struk <tadeusz.struk@intel.com>