lib/crypto: blake2s: Drop excessive const & rename block => data

lib/crypto: blake2s: Drop excessive const & rename block => data

A couple more small cleanups to the BLAKE2s code before these things get
propagated into the BLAKE2b code:

- Drop 'const' from some non-pointer function parameters.  It was a bit
  excessive and not conventional.

- Rename 'block' argument of blake2s_compress*() to 'data'.  This is for
  consistency with the SHA-* code, and also to avoid the implication
  that it points to a singular "block".

No functional changes.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251018043106.375964-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>