crypto: authenc - Avoid using clobbered request pointer

Authenc works in two stages for encryption, it first encrypts and
then computes an ICV.  The context memory of the request is used
by both operations.  The problem is that when an asynchronous
encryption completes, we will compute the ICV and then reread the
context memory of the encryption to get the original request.

It just happens that we have a buffer of 16 bytes in front of the
request pointer, so ICVs of 16 bytes (such as SHA1) do not trigger
the bug.  However, any attempt to uses a larger ICV instantly kills
the machine when the first asynchronous encryption is completed.

This patch fixes this by saving the request pointer before we start
the ICV computation.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>