crypto: xchacha20 - fix comments for test vectors

The kernel's ChaCha20 uses the RFC7539 convention of the nonce being 12
bytes rather than 8, so actually I only appended 12 random bytes (not
16) to its test vectors to form 24-byte nonces for the XChaCha20 test
vectors.  The other 4 bytes were just from zero-padding the stream
position to 8 bytes.  Fix the comments above the test vectors.

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