MIPS: smp: optimization for flush_tlb_mm when exiting

When process exits or execute new binary, it will call function
exit_mmap with old mm, there is such function call trace:
  exit_mmap(struct mm_struct *mm)
      --> tlb_finish_mmu(&tlb, 0, -1)
         --> arch_tlb_finish_mmu(tlb, start, end, force)
	    --> tlb_flush_mmu(tlb);
               --> tlb_flush(struct mmu_gather *tlb)
                  --> flush_tlb_mm(tlb->mm)

It is not necessary to flush tlb since oldmm is not used anymore
by the process, there is similar operations on IA64/ARM64 etc,
this patch adds such optimization on MIPS.
Signed-off-by: Mao Bibo <maobibo@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/smp.c

@@ -518,6 +518,12 @@ static inline void smp_on_each_tlb(void (*func) (void *info), void *info)
 
 void flush_tlb_mm(struct mm_struct *mm)
 {
+	if (!mm)
+		return;
+
+	if (atomic_read(&mm->mm_users) == 0)
+		return;		/* happens as a result of exit_mmap() */
+
 	preempt_disable();
 
 	if (cpu_has_mmid) {