nfs: disable leases over NFS

As Peter Staubach says elsewhere
(http://marc.info/?l=linux-kernel&m=118113649526444&w=2):

> The problem is that some file system such as NFSv2 and NFSv3 do
> not have sufficient support to be able to support leases correctly.
> In particular for these two file systems, there is no over the wire
> protocol support.
>
> Currently, these two file systems fail the fcntl(F_SETLEASE) call
> accidentally, due to a reference counting difference.  These file
> systems should fail more consciously, with a proper error to
> indicate that the call is invalid for them.

Define an nfs setlease method that just returns -EINVAL.

If someone can demonstrate a real need, perhaps we could reenable
them in the presence of the "nolock" mount option.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Cc: Peter Staubach <staubach@redhat.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/file.c

@@ -53,6 +53,7 @@ static int  nfs_fsync(struct file *, struct dentry *dentry, int datasync);
 static int nfs_check_flags(int flags);
 static int nfs_lock(struct file *filp, int cmd, struct file_lock *fl);
 static int nfs_flock(struct file *filp, int cmd, struct file_lock *fl);
+static int nfs_setlease(struct file *file, long arg, struct file_lock **fl);
 
 const struct file_operations nfs_file_operations = {
 	.llseek		= nfs_file_llseek,
@@ -69,6 +70,7 @@ const struct file_operations nfs_file_operations = {
 	.flock		= nfs_flock,
 	.splice_read	= nfs_file_splice_read,
 	.check_flags	= nfs_check_flags,
+	.setlease	= nfs_setlease,
 };
 
 const struct inode_operations nfs_file_inode_operations = {
@@ -558,3 +560,13 @@ static int nfs_flock(struct file *filp, int cmd, struct file_lock *fl)
 		return do_unlk(filp, cmd, fl);
 	return do_setlk(filp, cmd, fl);
 }
+
+static int nfs_setlease(struct file *file, long arg, struct file_lock **fl)
+{
+	/*
+	 * There is no protocol support for leases, so we have no way
+	 * to implement them correctly in the face of opens by other
+	 * clients.
+	 */
+	return -EINVAL;
+}