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

Now {pmd,pte}_mkdirty() set _PAGE_DIRTY bit unconditionally, this causes
random segmentation fault after commit 0ccf7f16 ("mm/thp: carry
over dirty bit when thp splits on pmd").

The reason is: when fork(), parent process use pmd_wrprotect() to clear
huge page's _PAGE_WRITE and _PAGE_DIRTY (for COW); then pte_mkdirty() set
_PAGE_DIRTY as well as _PAGE_MODIFIED while splitting dirty huge pages;
once _PAGE_DIRTY is set, there will be no tlb modify exception so the COW
machanism fails; and at last memory corruption occurred between parent
and child processes.

So, we should set _PAGE_DIRTY only when _PAGE_WRITE is set in {pmd,pte}_
mkdirty().

Cc: stable@vger.kernel.org
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/include/asm/pgtable.h

@@ -349,7 +349,9 @@ static inline pte_t pte_mkclean(pte_t pte)
 
 static inline pte_t pte_mkdirty(pte_t pte)
 {
-	pte_val(pte) |= (_PAGE_DIRTY | _PAGE_MODIFIED);
+	pte_val(pte) |= _PAGE_MODIFIED;
+	if (pte_val(pte) & _PAGE_WRITE)
+		pte_val(pte) |= _PAGE_DIRTY;
 	return pte;
 }
 
@@ -478,7 +480,9 @@ static inline pmd_t pmd_mkclean(pmd_t pmd)
 
 static inline pmd_t pmd_mkdirty(pmd_t pmd)
 {
-	pmd_val(pmd) |= (_PAGE_DIRTY | _PAGE_MODIFIED);
+	pmd_val(pmd) |= _PAGE_MODIFIED;
+	if (pmd_val(pmd) & _PAGE_WRITE)
+		pmd_val(pmd) |= _PAGE_DIRTY;
 	return pmd;
 }