Fix bug in scrypt KDF provider dup method

The scrypt KDF provider's dup method calls kdf_scrypt_new passing a
libctx, but a provider context is expected. Since the provider context
is passed as void *, this was not caught.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17873)
