nfs: rename pgio header ds_idx to ds_commit_idx

'ds_commit_idx' is a better name - it is used to select the right
commit bucket for pnfs.

Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index 4fd7793..5bc99f0 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -1328,7 +1328,7 @@
 	__u64			mds_offset;	/* Filelayout dense stripe */
 	struct nfs_page_array	page_array;
 	struct nfs_client	*ds_clp;	/* pNFS data server */
-	int			ds_idx;		/* ds index if ds_clp is set */
+	int			ds_commit_idx;	/* ds index if ds_clp is set */
 };
 
 struct nfs_mds_commit_info {