powerpc/mm/nohash: Ignore NULL stale_map entries

This happens with threads that are offline due to CPU hotplug
(including threads that were never "plugged in" to begin with because
SMT is disabled).
Signed-off-by: NScott Wood <scottwood@freescale.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mmu_context_nohash.c

@@ -112,8 +112,10 @@ static unsigned int steal_context_smp(unsigned int id)
 		 */
 		for_each_cpu(cpu, mm_cpumask(mm)) {
 			for (i = cpu_first_thread_sibling(cpu);
-			     i <= cpu_last_thread_sibling(cpu); i++)
+			     i <= cpu_last_thread_sibling(cpu); i++) {
+				if (stale_map[i])
 					__set_bit(id, stale_map[i]);
+			}
 			cpu = i - 1;
 		}
 		return id;
@@ -272,6 +274,7 @@ void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
 		/* XXX This clear should ultimately be part of local_flush_tlb_mm */
 		for (i = cpu_first_thread_sibling(cpu);
 		     i <= cpu_last_thread_sibling(cpu); i++) {
+			if (stale_map[i])
 				__clear_bit(id, stale_map[i]);
 		}
 	}