rand: add callbacks to cleanup the user entropy resp. nonce

rand: add callbacks to cleanup the user entropy resp. nonce

The `get_user_{entropy,nonce}` callbacks were add recently to the
dispatch table in commit 4cde7585ce8e. Instead of adding corresponding
`cleanup_user_{entropy,nonce}` callbacks, the `cleanup_{entropy,nonce}`
callbacks were reused. This can cause a problem in the case where the
seed source is replaced by a provider: the buffer gets allocated by
the provider but cleared by the core.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22423)