erofs: avoid memory allocation failure during rolling decompression

Currently, err would be treated as io error. Therefore, it'd be
better to ensure memory allocation during rolling decompression
to avoid such io error.

In the long term, we might consider adding another !Uptodate case
for such case.

Link: https://lore.kernel.org/r/20210316031515.90954-1-huangjianan@oppo.comReviewed-by: NGao Xiang <hsiangkao@redhat.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NHuang Jianan <huangjianan@oppo.com>
Signed-off-by: NGuo Weichao <guoweichao@oppo.com>
Signed-off-by: NGao Xiang <hsiangkao@redhat.com>

fs/erofs/decompressor.c

@@ -73,9 +73,8 @@ static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
 			victim = availables[--top];
 			get_page(victim);
 		} else {
-			victim = erofs_allocpage(pagepool, GFP_KERNEL);
-			if (!victim)
-				return -ENOMEM;
+			victim = erofs_allocpage(pagepool,
+						 GFP_KERNEL | __GFP_NOFAIL);
 			set_page_private(victim, Z_EROFS_SHORTLIVED_PAGE);
 		}
 		rq->out[i] = victim;