passwd.c: use the actual ROUNDS_DEFAULT macro

passwd.c: use the actual ROUNDS_DEFAULT macro

Before this commit, the `ROUNDS_DEFAULT` macro was not used at all, while defined in the source code.
Instead, a `unsigned int rounds = 5000;` was set, which uses the same value.

This commit changes the `5000` to `ROUNDS_DEFAULT`.

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14156)