This reverts 35a33ff3 ("random: use memmove instead of memcpy for
remaining 32 bytes"), which was made on a totally bogus basis. The thing
it was worried about overlapping came from the stack, not from one of
its arguments, as Eric pointed out.

But the fact that this confusion even happened draws attention to the
fact that it's a bit non-obvious that the random_data parameter can
alias chacha_state, and in fact should do so when the caller can't rely
on the stack being cleared in a timely manner. So this commit documents
that.
Reported-by: NEric Biggers <ebiggers@kernel.org>
Reviewed-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>