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/nfs4proc.c b/fs/nfs/nfs4proc.c
index 21cd1f2..4794ca6 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -4055,7 +4055,7 @@
 }
 
 static bool nfs4_read_stateid_changed(struct rpc_task *task,
-		struct nfs_readargs *args)
+		struct nfs_pgio_args *args)
 {
 
 	if (!nfs4_error_stateid_expired(task->tk_status) ||
@@ -4121,7 +4121,7 @@
 }
 
 static bool nfs4_write_stateid_changed(struct rpc_task *task,
-		struct nfs_writeargs *args)
+		struct nfs_pgio_args *args)
 {
 
 	if (!nfs4_error_stateid_expired(task->tk_status) ||