Fixup bugs related to nfsv4 stable restart file.

1) If there's no system dataset when the system is booted don't try
   to create the nfs-stablerestart symlinks.  That path lies madness.
   (and a traceback in the notifier at boot)

2) On the other hand, if a pool gets created, go ahead and create the
   symlinks that the nfsv4 server will need when the system dataset
   is created.