arm64: cpufeature: Fix CPU_OUT_OF_SPEC taint for uniform systems

Commit 3fde2999 ("arm64: cpufeature: Don't dump useless backtrace on
CPU_OUT_OF_SPEC") changed the cpufeature detection code to use add_taint
instead of WARN_TAINT_ONCE when detecting a heterogeneous system with
mismatched feature support. Unfortunately, this resulted in all systems
getting the taint, regardless of any feature mismatch.

This patch fixes the problem by conditionalising the taint on detecting
a feature mismatch.
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/cpufeature.c

@@ -639,8 +639,10 @@ void update_cpu_features(int cpu,
 	 * Mismatched CPU features are a recipe for disaster. Don't even
 	 * pretend to support them.
 	 */
+	if (taint) {
 		pr_warn_once("Unsupported CPU feature variation detected.\n");
 		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+	}
 }
 
 u64 read_sanitised_ftr_reg(u32 id)