crypto: rsa-pkcs1pad - simplify error handling in pkcs1pad_create()

Simplify the error handling in pkcs1pad_create() by taking advantage of
crypto_grab_akcipher() now handling an ERR_PTR() name and by taking
advantage of crypto_drop_akcipher() now accepting (as a no-op) a spawn
that hasn't been grabbed yet.

While we're at it, also simplify the way the hash_name optional argument
is handled.  We only need to check whether it's present in one place,
and we can just assign directly to ctx->digest_info.

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