powerpc/mm: Optimize detection of thread local mm's

Instead of comparing the whole CPU mask every time, let's
keep a counter of how many bits are set in the mask. Thus
testing for a local mm only requires testing if that counter
is 1 and the current CPU bit is set in the mask.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/mmu_context_book3s64.c

@@ -170,6 +170,8 @@ int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 #ifdef CONFIG_SPAPR_TCE_IOMMU
 	mm_iommu_init(mm);
 #endif
+	atomic_set(&mm->context.active_cpus, 0);
+
 	return 0;
 }