Make the pte unmapping atomic wrt other threads.

mm/memory.c

@@ -343,8 +343,7 @@ static void zap_pte_range(mmu_gather_t *tlb, pmd_t * pmd, unsigned long address,
 		if (pte_present(pte)) {
 			unsigned long pfn = pte_pfn(pte);
 
-			pte_clear(ptep);
-			pfn = pte_pfn(pte);
+			pte = ptep_get_and_clear(ptep);
 			tlb_remove_tlb_entry(tlb, pte, address+offset);
 			if (pfn_valid(pfn)) {
 				struct page *page = pfn_to_page(pfn);