selftests/futex: Refactor futex_priv_hash with kselftest_harness.h

To reduce the boilerplate code, refactor futex_priv_hash test to use
kselftest_harness header instead of futex's logging header.

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>