crypto: shash - Handle partial blocks in API

crypto: shash - Handle partial blocks in API

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

Rather than duplicating the partial block handling many times,
add this functionality to the shash API.

It is optional (e.g., hmac would never need this by relying on
the partial block handling of the underlying hash), and to enable
it set the bit CRYPTO_AHASH_ALG_BLOCK_ONLY.

The export format is always that of the underlying hash export,
plus the partial block buffer, followed by a single-byte for the
partial block length.

Set the bit CRYPTO_AHASH_ALG_FINAL_NONZERO to withhold an extra
byte in the partial block.  This will come in handy when this
is extended to ahash where hardware often can't deal with a
zero-length final.

It will also be used for algorithms requiring an extra block for
finalisation (e.g., cmac).

As an optimisation, set the bit CRYPTO_AHASH_ALG_FINUP_MAX if
the algorithm wishes to get as much data as possible instead of
just the last partial block.

The descriptor will be zeroed after finalisation.

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