chacha: add variant that treats counter value as 32-bit

chacha: add variant that treats counter value as 32-bit

The ChaCha-Poly1305 implementation previously used the chacha_crypt
function that assumes the block counter is 64-bit long, while RFC 8439
defines that the counter is 32-bit long.  Although this should be fine
as long as up to 256 gigabytes of data is encrypted with the same key,
it would be nice to use a separate functions (chacha_set_counter32 and
chacha_crypt32) that assume the counter is 32-bit long.

Signed-off-by: Daiki Ueno <dueno@redhat.com>