powerpc/mm: bail out early when flushing TLB page

MMU_NO_CONTEXT is conditionally defined as 0 or (unsigned int)-1. However,
in __flush_tlb_page() a corresponding variable is only tested for open
coded 0, which can cause NULL pointer dereference if `mm' argument was
legitimately passed as such.

Bail out early in case the first argument is NULL, thus eliminate confusion
between different values of MMU_NO_CONTEXT and avoid disabling and then
re-enabling preemption unnecessarily.
Signed-off-by: NArseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: NScott Wood <scottwood@freescale.com>

arch/powerpc/mm/tlb_nohash.c

@@ -284,8 +284,11 @@ void __flush_tlb_page(struct mm_struct *mm, unsigned long vmaddr,
 	struct cpumask *cpu_mask;
 	unsigned int pid;
 
+	if (unlikely(!mm))
+		return;
+
 	preempt_disable();
-	pid = mm ? mm->context.id : 0;
+	pid = mm->context.id;
 	if (unlikely(pid == MMU_NO_CONTEXT))
 		goto bail;
 	cpu_mask = mm_cpumask(mm);