pnfs: encode_layoutreturn

Add a layout driver method to encode the layout type specific
opaque part of layout return in-line in the xdr stream.

Currently the pnfs-objects layout driver uses it to encode i/o error
information on LAYOUTRETURN.

Signed-off-by: Andy Adamson <andros@netapp.com>
[fixup layout header pointer for encode_layoutreturn]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index af3967a..1b6b207 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -104,6 +104,10 @@
 	enum pnfs_try_status (*write_pagelist) (struct nfs_write_data *nfs_data, int how);
 
 	void (*free_deviceid_node) (struct nfs4_deviceid_node *);
+
+	void (*encode_layoutreturn) (struct pnfs_layout_hdr *layoutid,
+				     struct xdr_stream *xdr,
+				     const struct nfs4_layoutreturn_args *args);
 };
 
 struct pnfs_layout_hdr {