virtio-mem: silence a static checker warning

Smatch complains that "rc" can be uninitialized if we hit the "break;"
statement on the first iteration through the loop.  I suspect that this
can't happen in real life, but returning a zero literal is cleaner and
silence the static checker warning.

Fixes: 5f1f79bb ("virtio-mem: Paravirtualized memory hotplug")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200610085911.GC5439@mwandaSigned-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_mem.c

@@ -1192,7 +1192,7 @@ static int virtio_mem_mb_plug_any_sb(struct virtio_mem *vm, unsigned long mb_id,
 						VIRTIO_MEM_MB_STATE_OFFLINE);
 	}
 
-	return rc;
+	return 0;
 }
 
 /*