webrev: http://cr.illumos.org/~webrev/marcel/il-zfs-mnt-protect/
This adds a protection of empty ZFS mount points to NFSv2/3 server implementation.

I tested all NFS operations from both NFSv2 and NFSv3 clients to make sure the

fix works as expected.  I confirmed that the NFS server protects empty ZFS

mount points only (with the access denied error).  Non-empty ZFS mount points

and mount points for other filesystems are not protected.  I also tested basic

NFSv4 functionality to make sure there is no regression introduced.