drbg: provide requested amount of entropy, rather than self-strength

drbg: provide requested amount of entropy, rather than self-strength

Parent DRBG can be seed source (os or jitter) and thus able to provide
unlimited entropy.

get_entropy is documented to provide at least the request amount of
entropy. If requested amount of entropy is same as, or less than
drbg->strength, everything is compliant. However, if requested entropy
is more than drbg->strength (unlikely, but possible), the returned
amount of entropy will be insufficient and additional repeated calls
to get_entropy will be required.

Reading history of refactors, it seems to me that this function call
previouslly had assumptions and usecases that couldn't ever request or
require more than strength amount of entropy.

If entropy is set, request that amount, otherwise request
drbg->strength amount.

Reviewed-by: Hugo Landau <hlandau@devever.net>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/25850)