crypto: lib/chacha - add strongly-typed state zeroization

Now that the ChaCha state matrix is strongly-typed, add a helper
function chacha_zeroize_state() which zeroizes it.  Then convert all
applicable callers to use it instead of direct memzero_explicit.  No
functional changes.

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