crypto: chacha - constify ctx and iv arguments

Constify the ctx and iv arguments to crypto_chacha_init() and the
various chacha*_stream_xor() functions.  This makes it clear that they
are not modified.

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

arch/arm/crypto/chacha-neon-glue.c

@@ -63,7 +63,7 @@ static void chacha_doneon(u32 *state, u8 *dst, const u8 *src,
 }
 
 static int chacha_neon_stream_xor(struct skcipher_request *req,
-				  struct chacha_ctx *ctx, u8 *iv)
+				  const struct chacha_ctx *ctx, const u8 *iv)
 {
 	struct skcipher_walk walk;
 	u32 state[16];