crypto: aead - Document behaviour of AD in destination buffer

This patch defines the behaviour of AD in the new interface more
clearly.  In particular, it specifies that if the user must copy
the AD to the destination manually when src != dst if they wish
to guarantee that the destination buffer contains a copy of the
AD.

The reason for this is that otherwise every AEAD implementation
would have to perform such a copy when src != dst.  In reality
most users do in-place processing where src == dst so this is
not an issue.

This patch also kills some remaining references to cryptoff.

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