Keep read and write fds with each nlm_file

We shouldn't really be using a read-only file descriptor to take a write
lock.

Most filesystems will put up with it.  But NFS, for example, won't.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/lockd/bind.h

@@ -27,7 +27,8 @@ struct rpc_task;
 struct nlmsvc_binding {
 	__be32			(*fopen)(struct svc_rqst *,
 						struct nfs_fh *,
-						struct file **);
+						struct file **,
+						int mode);
 	void			(*fclose)(struct file *);
 };