erofs: insert to managed cache after adding to pcl

Previously, it could be some concern to call add_to_page_cache_lru()
with page->mapping == Z_EROFS_MAPPING_STAGING (!= NULL).

In contrast, page->private is used instead now, so partially revert
commit 5ddcee1f ("erofs: get rid of __stagingpage_alloc helper")
with some adaption for simplicity.

Link: https://lore.kernel.org/r/20201208095834.3133565-2-hsiangkao@redhat.comReviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

fs/erofs/zdata.c

@@ -1065,29 +1065,21 @@ static struct page *pickup_page_for_submission(struct z_erofs_pcluster *pcl,
 	put_page(page);
 out_allocpage:
 	page = erofs_allocpage(pagepool, gfp | __GFP_NOFAIL);
-	if (!tocache || add_to_page_cache_lru(page, mc, index + nr, gfp)) {
-		/* turn into temporary page if fails */
-		set_page_private(page, Z_EROFS_SHORTLIVED_PAGE);
-		tocache = false;
-	}
-
 	if (oldpage != cmpxchg(&pcl->compressed_pages[nr], oldpage, page)) {
-		if (tocache) {
-			/* since it added to managed cache successfully */
-			unlock_page(page);
-			put_page(page);
-		} else {
-			list_add(&page->lru, pagepool);
-		}
+		list_add(&page->lru, pagepool);
 		cond_resched();
 		goto repeat;
 	}
 
-	if (tocache) {
-		attach_page_private(page, pcl);
-		/* drop a ref added by allocpage (then we have 2 refs here) */
-		put_page(page);
+	if (!tocache || add_to_page_cache_lru(page, mc, index + nr, gfp)) {
+		/* turn into temporary page if fails (1 ref) */
+		set_page_private(page, Z_EROFS_SHORTLIVED_PAGE);
+		goto out;
 	}
+	attach_page_private(page, pcl);
+	/* drop a refcount added by allocpage (then we have 2 refs here) */
+	put_page(page);
+
 out:	/* the only exit (for tracing and debugging) */
 	return page;
 }