nfs: fix duplicate proc entries

Commit 65b38851
("NFS: Fix /proc/fs/nfsfs/servers and /proc/fs/nfsfs/volumes")

updated the following function:
static int nfs_volume_list_open(struct inode *inode, struct file *file)

it used &nfs_server_list_ops instead of &nfs_volume_list_ops
which means cat /proc/fs/nfsfs/volumes = /proc/fs/nfsfs/servers
Signed-off-by: NFabian Frederick <fabf@skynet.be>
Fixes: 65b38851 (NFS: Fix /proc/fs/nfsfs/servers and...)
Cc: stable@vger.kernel.org # 3.4.x+
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/client.c

@@ -1318,7 +1318,7 @@ static int nfs_server_list_show(struct seq_file *m, void *v)
  */
 static int nfs_volume_list_open(struct inode *inode, struct file *file)
 {
-	return seq_open_net(inode, file, &nfs_server_list_ops,
+	return seq_open_net(inode, file, &nfs_volume_list_ops,
 			   sizeof(struct seq_net_private));
 }