crypto: rsa-pkcs1pad - use clearer variable names

The new convention for akcipher_alg::verify makes it unclear which
values are the lengths of the signature and digest.  Add local variables
to make it clearer what is going on.

Also rename the digest_size variable in pkcs1pad_sign(), as it is
actually the digest *info* size, not the digest size which is different.

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