NFS server File Handle changing upon reboot

Hiroki Sato spotted a problem with NFS server file handles (FHs)
changing after a server upgrade, because the exported file
system type(s) get configured in a different order and, therefore,
assigned different vfs_typenum values.

A patch has been worked out, after discussion with various folks,
that uses a hash function to assign the vfs_typenum values.

This fixes the problem, except it has one downside:
- The first server boot after the patch has been applied will
  result in FHs changing and, as such, NFS clients
  will need to remount after this upgrade.

So, finally to why I am posting, which is to ask for opinions
on what should be done with this patch?
1 - Ask re_at_ for permission to commit this to -current for 9.0,
    so that the FH change happens at the 8.X->9.0 upgrade.
    (It does seem that if some variant of this should go in, then
     a major release seems like the correct time to do it?)
2 - Add a loader.conf variable to the patch, which would allow
    a sysadmin to "flip the switch" when it is convenient for
    them. (I do have a concern that this might just cause more
    confusion w.r.t. when/what needs to be done.)
3 - Do #2, for 8.X and make the patch the default for 9.0.
4 - Forget the patch and leave things the way the are now.

Please respond asap, since if it is to go in head/9.0, I need
to queue up the patch soon.

Thanks in advance for any comments, rick
ps: This patch was discussed under the subject
    "fsid change of ZFS?"