crypto: blake2s - optimize blake2s initialization

If no key was provided, then don't waste time initializing the block
buffer, as its initial contents won't be used.

Also, make crypto_blake2s_init() and blake2s() call a single internal
function __blake2s_init() which treats the key as optional, rather than
conditionally calling blake2s_init() or blake2s_init_key().  This
reduces the compiled code size, as previously both blake2s_init() and
blake2s_init_key() were being inlined into these two callers, except
when the key size passed to blake2s() was a compile-time constant.

These optimizations aren't that significant for BLAKE2s.  However, the
equivalent optimizations will be more significant for BLAKE2b, as
everything is twice as big in BLAKE2b.  And it's good to keep things
consistent rather than making optimizations for BLAKE2b but not BLAKE2s.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>