LoongArch: Set _PAGE_DIRTY only if _PAGE_MODIFIED is set in {pmd,pte}_mkwrite()

Set _PAGE_DIRTY only if _PAGE_MODIFIED is set in {pmd,pte}_mkwrite().
Otherwise, _PAGE_DIRTY silences the TLB modify exception and make us
have no chance to mark a pmd/pte dirty (_PAGE_MODIFIED) for software.
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/pgtable.h

@@ -357,7 +357,9 @@ static inline pte_t pte_mkdirty(pte_t pte)
 
 static inline pte_t pte_mkwrite(pte_t pte)
 {
-	pte_val(pte) |= (_PAGE_WRITE | _PAGE_DIRTY);
+	pte_val(pte) |= _PAGE_WRITE;
+	if (pte_val(pte) & _PAGE_MODIFIED)
+		pte_val(pte) |= _PAGE_DIRTY;
 	return pte;
 }
 
@@ -457,7 +459,9 @@ static inline int pmd_write(pmd_t pmd)
 
 static inline pmd_t pmd_mkwrite(pmd_t pmd)
 {
-	pmd_val(pmd) |= (_PAGE_WRITE | _PAGE_DIRTY);
+	pmd_val(pmd) |= _PAGE_WRITE;
+	if (pmd_val(pmd) & _PAGE_MODIFIED)
+		pmd_val(pmd) |= _PAGE_DIRTY;
 	return pmd;
 }