kernel - Force NFSv3 for diskless nfs mount

kernel - Force NFSv3 for diskless nfs mount

* Ignore any file handles passed from bootp.  These are NFSv2 handles
  and NFSv2 just doesn't have the directory support needed if the
  server is running a filesystem with 64-bit directory cookies (as HAMMER
  does).

  This will force the kernel's diskless nfs mount to re-authorize and
  acquire a NFSv3 handle for the root mount.

* Undo a bit of a previous commit where I tried to enable readdirplus for
  NFSv2 to workaround NFSv2 directory cookie issues.  It just doesn't work.
  readdirplus is again force-disabled for NFSv2 mounts.