EVP: Let EVP_PKEY_gen() initialize ctx->keygen_info

EVP: Let EVP_PKEY_gen() initialize ctx->keygen_info

In EVP_PKEY_METHOD code, the backend initializes ctx->keygen_info.
With provider side code, it's not possible to reach back into the
EVP_PKEY_CTX in the same manner, so we need to make that
initialization in the central generation function, EVP_PKEY_gen().

This isn't quite compatible with the idea that keygen_info could have
an arbitrary amount of elements, but since all our legacy backends use
exactly two elements, that's what we go for.

Fixes #12047

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/12048)