kselftest/harness: Use helper to avoid zero-size memset warning

kselftest/harness: Use helper to avoid zero-size memset warning

When building kselftests with a toolchain that enables source
fortification (e.g., Android's build environment, which uses
-D_FORTIFY_SOURCE=3), a build failure occurs in tests that use an
empty FIXTURE().

The root cause is that an empty fixture struct results in
`sizeof(self_private)` evaluating to 0. The compiler's fortification
checks then detect the `memset()` call with a compile-time constant size
of 0, issuing a `-Wuser-defined-warnings` which is promoted to an error
by `-Werror`.

An initial attempt to guard the call with `if (sizeof(self_private) > 0)`
was insufficient. The compiler's static analysis is aggressive enough
to flag the `memset(..., 0)` pattern before evaluating the conditional,
thus still triggering the error.

To resolve this robustly, this change introduces a `static inline`
helper function, `__kselftest_memset_safe()`. This function wraps the
size check and the `memset()` call. By replacing the direct `memset()`
in the `__TEST_F_IMPL` macro with a call to this helper, we create an
abstraction boundary. This prevents the compiler's static analyzer from
"seeing" the problematic pattern at the macro expansion site, resolving
the build failure.

Build Context:
Compiler: Android (14488419, +pgo, +bolt, +lto, +mlgo, based on r584948) clang version 22.0.0 (https://android.googlesource.com/toolchain/llvm-project 2d65e4108033380e6fe8e08b1f1826cd2bfb0c99)
Relevant Options: -O2 -Wall -Werror -D_FORTIFY_SOURCE=3 -target i686-linux-android10000

Test: m kselftest_futex_futex_requeue_pi

Removed Gerrit Change-Id
Shuah Khan <skhan@linuxfoundation.org>

Link: https://lore.kernel.org/r/20251224084120.249417-1-wakel@google.com
Signed-off-by: Wake Liu <wakel@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>