crypto: chacha20poly1305 - a few cleanups

- Use sg_init_one() instead of sg_init_table() then sg_set_buf().

- Remove unneeded calls to sg_init_table() prior to scatterwalk_ffwd().

- Simplify initializing the poly tail block.

- Simplify computing padlen.

This doesn't change any actual behavior.

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