constify dentry_operations: NFS

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index 2e5f000..43a713f 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -785,7 +785,7 @@
  */
 struct nfs_rpc_ops {
 	u32	version;		/* Protocol version */
-	struct dentry_operations *dentry_ops;
+	const struct dentry_operations *dentry_ops;
 	const struct inode_operations *dir_inode_ops;
 	const struct inode_operations *file_inode_ops;