NFS: Create a common argument structure for reads and writes

Reads and writes have very similar arguments.  This patch combines them
together and documents the few fields used only by write.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/read.c b/fs/nfs/read.c
index 7f87461..46d5552 100644
--- a/fs/nfs/read.c
+++ b/fs/nfs/read.c
@@ -470,7 +470,7 @@
 
 static void nfs_readpage_retry(struct rpc_task *task, struct nfs_read_data *data)
 {
-	struct nfs_readargs *argp = &data->args;
+	struct nfs_pgio_args *argp = &data->args;
 	struct nfs_readres *resp = &data->res;
 
 	/* This is a short read! */