Comment # 10 on bug 1120087 from Neil Brown

> I would be a lot happier if nfsd did survive a file system unmount.

A proper solution for that will be a while in coming.

You may think this solution is worse than the disease, but one option to keep
nfsd running it to remove the export from /etc/exports and request the export
manually whenever you boot the system.
i.e. run
  exportfs -o option,option,... client:/path/to/export

This will tell nfsd about the export, but systemd won't know about the
dependency.