NFSD: Fix returned READDIR offset cookie

Code inspection shows that the server's NFSv3 READDIR implementation
handles offset cookies slightly differently than the NFSv2 READDIR,
NFSv3 READDIRPLUS, and NFSv4 READDIR implementations,
and there doesn't seem to be any need for this difference.

As a clean up, I copied the logic from nfsd3_proc_readdirplus().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>