selftests/futex: Fix format-security warnings in futex_priv_hash

Fix format-security warnings by using proper format strings when passing
message variables to ksft_exit_fail_msg(), ksft_test_result_pass(), and
ksft_test_result_skip() function.

Thus prevent potential security issues and eliminate compiler warnings when
building with -Wformat-security.

Signed-off-by: Nai-Chen Cheng <bleach1827@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250827130011.677600-4-bigeasy@linutronix.de