nfs: fix redundant readdir request after get eof

When a directory contains 17 files (except . and ..), nfs client sends
a redundant readdir request after get eof.

A simple reproduce,
At NFS server, create a directory with 17 files under exported directory.
 # mkdir test
 # cd test
 # for i in {0..16}  ; do touch $i; done

At NFS client, no matter mounting through nfsv3 or nfsv4,
does ls (or ll) at the created test directory.

A tshark output likes following (for nfsv4),

 # tshark -i eth0 tcp port 2049 -Tfields -e ip.src -e ip.dst -e nfs -e nfs.cookie4

srcip   dstip   SEQUENCE, PUTFH, READDIR        0
dstip   srcip   SEQUENCE PUTFH READDIR  909539109313539306,2108391201987888856,2305312124304486544,2566335452463141496,2978225129081509984,4263037479923412583,4304697173036510679,4666703455469210097,4759208201298769007,4776701232145978803,5338408478512081262,5949498658935544804,5971526429894832903,6294060338267709855,6528840566229532529,8600463293536422524,9223372036854775807
srcip   dstip
srcip   dstip   SEQUENCE, PUTFH, READDIR        9223372036854775807
dstip   srcip   SEQUENCE PUTFH READDIR

The READDIR with cookie 9223372036854775807(0x7FFFFFFFFFFFFFFF) is redundant.

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>