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/[email protected]
Signed-off-by: Qi Zheng <[email protected]>
Suggested-by: Bibo Mao <[email protected]>
Acked-by: Huacai Chen <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Qi Zheng

arch/loongarch/include/asm/pgtable.h

@@ -412,6 +412,9 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
 	__update_tlb(vma, address, ptep);
 }
 
+#define __HAVE_ARCH_UPDATE_MMU_TLB
+#define update_mmu_tlb	update_mmu_cache
+
 static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
 			unsigned long address, pmd_t *pmdp)
 {