nfsd4: rename nfs4_rpc_args->nfsd4_cb_args

With apologies for the gratuitous rename, the new name seems more
helpful to me.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 59313f1..f988b90 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -64,14 +64,14 @@
 	(s)->si_fileid, \
 	(s)->si_generation
 
-struct nfs4_rpc_args {
+struct nfsd4_cb_args {
 	void			*args_op;
 	struct nfs4_client	*args_clp;
 	u32			args_minorversion;
 };
 
 struct nfsd4_callback {
-	struct nfs4_rpc_args cb_args;
+	struct nfsd4_cb_args cb_args;
 	struct work_struct cb_work;
 };