LoongArch: update local TLB if PTE entry exists

Currently, the implementation of update_mmu_tlb() is empty if
__HAVE_ARCH_UPDATE_MMU_TLB is not defined.  Then if two threads
concurrently fault at the same page, the second thread that did not win
the race will give up and do nothing.  In the LoongArch architecture, this
second thread will trigger another fault, and only updates its local TLB.

Instead of triggering another fault, it's better to implement
update_mmu_tlb() to directly update the local TLB of the second thread. 
Just do it.

Link: https://lkml.kernel.org/r/20220929112318.32393-3-zhengqi.arch@bytedance.com


Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Suggested-by: Bibo Mao <maobibo@loongson.cn>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Cc: Chris Zankel <chris@zankel.net>
Cc: David Hildenbrand <david@redhat.com>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>