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>

fs/nfs/nfs4xdr.c

@@ -1920,8 +1920,13 @@ encode_layoutreturn(struct xdr_stream *xdr,
 	spin_lock(&args->inode->i_lock);
 	xdr_encode_opaque_fixed(p, &args->stateid.data, NFS4_STATEID_SIZE);
 	spin_unlock(&args->inode->i_lock);
+	if (NFS_SERVER(args->inode)->pnfs_curr_ld->encode_layoutreturn) {
+		NFS_SERVER(args->inode)->pnfs_curr_ld->encode_layoutreturn(
+			NFS_I(args->inode)->layout, xdr, args);
+	} else {
 		p = reserve_space(xdr, 4);
 		*p = cpu_to_be32(0);
+	}
 	hdr->nops++;
 	hdr->replen += decode_layoutreturn_maxsz;
 }

fs/nfs/pnfs.h

@@ -104,6 +104,10 @@ struct pnfs_layoutdriver_type {
 	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 {