crypto: ahash - Handle partial blocks in API

Provide an option to handle the partial blocks in the ahash API.
Almost every hash algorithm has a block size and are only able
to hash partial blocks on finalisation.

As a first step disable virtual address support for algorithms
with state sizes larger than HASH_MAX_STATESIZE.  This is OK as
virtual addresses are currently only used on synchronous fallbacks.

This means ahash_do_req_chain only needs to handle synchronous
fallbacks, removing the complexities of saving the request state.

Also move the saved request state into the ahash_request object
as nesting is no longer possible.

Add a scatterlist to ahash_request to store the partial block.

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