erofs: shouldn't churn the mapping page for duplicated copies

If other duplicated copies exist in one decompression shot, should
leave the old page as is rather than replace it with the new duplicated
one.  Otherwise, the following cold path to deal with duplicated copies
will use the invalid bvec.  It impacts compressed data deduplication.

Also, shift the onlinepage EIO bit to avoid touching the signed bit.

Fixes: 267f2492 ("erofs: introduce multi-reference pclusters (fully-referenced)")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221012045056.13421-1-hsiangkao@linux.alibaba.com

fs/erofs/zdata.c

@@ -888,15 +888,13 @@ static void z_erofs_do_decompressed_bvec(struct z_erofs_decompress_backend *be,
 
 	if (!((bvec->offset + be->pcl->pageofs_out) & ~PAGE_MASK)) {
 		unsigned int pgnr;
-		struct page *oldpage;
 
 		pgnr = (bvec->offset + be->pcl->pageofs_out) >> PAGE_SHIFT;
 		DBG_BUGON(pgnr >= be->nr_pages);
-		oldpage = be->decompressed_pages[pgnr];
-		be->decompressed_pages[pgnr] = bvec->page;
-
-		if (!oldpage)
+		if (!be->decompressed_pages[pgnr]) {
+			be->decompressed_pages[pgnr] = bvec->page;
 			return;
+		}
 	}
 
 	/* (cold path) one pcluster is requested multiple times */

fs/erofs/zdata.h

@@ -126,10 +126,10 @@ static inline unsigned int z_erofs_pclusterpages(struct z_erofs_pcluster *pcl)
 }
 
 /*
- * bit 31: I/O error occurred on this page
- * bit 0 - 30: remaining parts to complete this page
+ * bit 30: I/O error occurred on this page
+ * bit 0 - 29: remaining parts to complete this page
  */
-#define Z_EROFS_PAGE_EIO			(1 << 31)
+#define Z_EROFS_PAGE_EIO			(1 << 30)
 
 static inline void z_erofs_onlinepage_init(struct page *page)
 {