OpenRC uses "umount -O _netdev" in the netmount script as part of the process to unmount all network file systems on Linux. However, the busybox umount command does not support -O. If it did, I believe we could just use this command to unmount *all* network file systems on Linux and not worry about fs types.
https://bugs.busybox.net/show_bug.cgi?id=8566