nfs41: add a helper function to set layoutcommit after commit

Track lwb in nfs_commit_data so that we can use it to setup
layoutcommit in commit_done callback.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index 0040629..e563b2c 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -1328,6 +1328,7 @@
 	struct pnfs_layout_segment *lseg;
 	struct nfs_client	*ds_clp;	/* pNFS data server */
 	int			ds_commit_index;
+	loff_t			lwb;
 	const struct rpc_call_ops *mds_ops;
 	const struct nfs_commit_completion_ops *completion_ops;
 	int (*commit_done_cb) (struct rpc_task *task, struct nfs_commit_data *data);