x86: warn if arch_flush_lazy_mmu_cpu is called in preemptible context

Impact: Catch cases where lazy MMU state is active in a preemtible context

arch_flush_lazy_mmu_cpu() has been changed to disable preemption so
the checks in enter/leave will never trigger. Put the preemtible()
check into arch_flush_lazy_mmu_cpu() to catch such cases.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/paravirt.c

@@ -273,6 +273,7 @@ void arch_flush_lazy_mmu_mode(void)
 	preempt_disable();
 
 	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
+		WARN_ON(preempt_count() == 1);
 		arch_leave_lazy_mmu_mode();
 		arch_enter_lazy_mmu_mode();
 	}
@@ -285,6 +286,7 @@ void arch_flush_lazy_cpu_mode(void)
 	preempt_disable();
 
 	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU) {
+		WARN_ON(preempt_count() == 1);
 		arch_leave_lazy_cpu_mode();
 		arch_enter_lazy_cpu_mode();
 	}