MIPS: mm: Avoid HTW stop/start when dropping an inactive mm

If drop_mmu_context() is called with an mm that is not currently active
on the local CPU then there's no need for us to stop & start a hardware
page table walker because it can't be fetching entries for the ASID
corresponding to the mm we're operating on.

Move the htw_stop() & htw_start() calls into the block which we run only
if the mm is currently active, in order to avoid the redundant work.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org

arch/mips/include/asm/mmu_context.h

@@ -184,17 +184,18 @@ drop_mmu_context(struct mm_struct *mm)
 	unsigned int cpu;
 
 	local_irq_save(flags);
-	htw_stop();
 
 	cpu = smp_processor_id();
 	if (cpumask_test_cpu(cpu, mm_cpumask(mm)))  {
+		htw_stop();
 		get_new_mmu_context(mm);
 		write_c0_entryhi(cpu_asid(cpu, mm));
+		htw_start();
 	} else {
 		/* will get a new context next time */
 		cpu_context(cpu, mm) = 0;
 	}
-	htw_start();
+
 	local_irq_restore(flags);
 }