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>