crypto: chacha20poly1305 - Annotate struct chachapoly_ctx with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
salt to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>