erofs: fix partially uninitialized misuse in z_erofs_onlinepage_fixup

Hongyu reported "id != index" in z_erofs_onlinepage_fixup() with
specific aarch64 environment easily, which wasn't shown before.

After digging into that, I found that high 32 bits of page->private
was set to 0xaaaaaaaa rather than 0 (due to z_erofs_onlinepage_init
behavior with specific compiler options). Actually we only use low
32 bits to keep the page information since page->private is only 4
bytes on most 32-bit platforms. However z_erofs_onlinepage_fixup()
uses the upper 32 bits by mistake.

Let's fix it now.
Reported-and-tested-by: Hongyu Jin <hongyu.jin@unisoc.com>
Fixes: 3883a79a ("staging: erofs: introduce VLE decompression support")
Cc: <stable@vger.kernel.org> # 4.19+
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200618234349.22553-1-hsiangkao@aol.comSigned-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/zdata.h

@@ -144,22 +144,22 @@ static inline void z_erofs_onlinepage_init(struct page *page)
 static inline void z_erofs_onlinepage_fixup(struct page *page,
 	uintptr_t index, bool down)
 {
-	unsigned long *p, o, v, id;
-repeat:
-	p = &page_private(page);
-	o = READ_ONCE(*p);
+	union z_erofs_onlinepage_converter u = { .v = &page_private(page) };
+	int orig, orig_index, val;
 
-	id = o >> Z_EROFS_ONLINEPAGE_INDEX_SHIFT;
-	if (id) {
+repeat:
+	orig = atomic_read(u.o);
+	orig_index = orig >> Z_EROFS_ONLINEPAGE_INDEX_SHIFT;
+	if (orig_index) {
 		if (!index)
 			return;
 
-		DBG_BUGON(id != index);
+		DBG_BUGON(orig_index != index);
 	}
 
-	v = (index << Z_EROFS_ONLINEPAGE_INDEX_SHIFT) |
-		((o & Z_EROFS_ONLINEPAGE_COUNT_MASK) + (unsigned int)down);
-	if (cmpxchg(p, o, v) != o)
+	val = (index << Z_EROFS_ONLINEPAGE_INDEX_SHIFT) |
+		((orig & Z_EROFS_ONLINEPAGE_COUNT_MASK) + (unsigned int)down);
+	if (atomic_cmpxchg(u.o, orig, val) != orig)
 		goto repeat;
 }