crypto: aead - Fix aead_instance struct size

The struct aead_instance is meant to extend struct crypto_instance
by incorporating the extra members of struct aead_alg.  However,
the current layout which is copied from shash/ahash does not specify
the struct fully.  In particular only aead_alg is present.

For shash/ahash this works because users there add extra headroom
to sizeof(struct crypto_instance) when allocating the instance.
Unfortunately for aead, this bit was lost when the new aead_instance
was added.

Rather than fixing it like shash/ahash, this patch simply expands
struct aead_instance to contain what is supposed to be there, i.e.,
adding struct crypto_instance.

In order to not break existing AEAD users, this is done through an
anonymous union.

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