powerpc/mm: Warn on flushing tlb page in kernel context

Function __flush_tlb_page() must only be called for user contexts, so
put in extra hardening to warn on calling it for kernel context.
Signed-off-by: NArseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/tlb_nohash.c

@@ -284,7 +284,11 @@ void __flush_tlb_page(struct mm_struct *mm, unsigned long vmaddr,
 	struct cpumask *cpu_mask;
 	unsigned int pid;
 
-	if (unlikely(!mm))
+	/*
+	 * This function as well as __local_flush_tlb_page() must only be called
+	 * for user contexts.
+	 */
+	if (unlikely(WARN_ON(!mm)))
 		return;
 
 	preempt_disable();