virtio-mem: convert most offline_and_remove_memory() errors to -EBUSY

Just like we do with alloc_contig_range(), let's convert all unknown
errors to -EBUSY, but WARN so we can look into the issue. For example,
offline_pages() could fail with -EINTR, which would be unexpected in our
case.
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230713145551.2824980-3-david@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -741,11 +741,15 @@ static int virtio_mem_offline_and_remove_memory(struct virtio_mem *vm,
 		 * immediately instead of waiting.
 		 */
 		virtio_mem_retry(vm);
-	} else {
-		dev_dbg(&vm->vdev->dev,
-			"offlining and removing memory failed: %d\n", rc);
+		return 0;
 	}
-	return rc;
+	dev_dbg(&vm->vdev->dev, "offlining and removing memory failed: %d\n", rc);
+	/*
+	 * We don't really expect this to fail, because we fake-offlined all
+	 * memory already. But it could fail in corner cases.
+	 */
+	WARN_ON_ONCE(rc != -ENOMEM && rc != -EBUSY);
+	return rc == -ENOMEM ? -ENOMEM : -EBUSY;
 }
 
 /*