crypto: blake2b - update file comment

The file comment for blake2b_generic.c makes it sound like it's the
reference implementation of BLAKE2b with only minor changes.  But it's
actually been changed a lot.  Update the comment to make this clearer.

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