crypto: lib/chacha - strongly type the ChaCha state

The ChaCha state matrix is 16 32-bit words.  Currently it is represented
in the code as a raw u32 array, or even just a pointer to u32.  This
weak typing is error-prone.  Instead, introduce struct chacha_state:

    struct chacha_state {
            u32 x[16];
    };

Convert all ChaCha and HChaCha functions to use struct chacha_state.
No functional changes.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>