Actually, I thought about doing this in sys/nfs/nfs_vfsops itself, to
   make old fstabs that only have rsize specified still work. If it is done
   there, amd doesn't need to do it anymore I guess.

   What do you think?

I think it's bogus.  We should stick with historic practice here.  
Giving the user an option to change the readdir request size is good,
but using it where we didn't before is asking for trouble.  There's no
*need* for this change.