NFSD: Add common helpers to decode void args and encode void results

Start off the conversion to xdr_stream by de-duplicating the functions
that decode void arguments and encode void results.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 20772f6..76300b0 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -3295,16 +3295,13 @@ static const char *nfsd4_op_name(unsigned opnum)
 	return "unknown_operation";
 }
 
-#define nfsd4_voidres			nfsd4_voidargs
-struct nfsd4_voidargs { int dummy; };
-
 static const struct svc_procedure nfsd_procedures4[2] = {
 	[NFSPROC4_NULL] = {
 		.pc_func = nfsd4_proc_null,
-		.pc_decode = nfs4svc_decode_voidarg,
-		.pc_encode = nfs4svc_encode_voidres,
-		.pc_argsize = sizeof(struct nfsd4_voidargs),
-		.pc_ressize = sizeof(struct nfsd4_voidres),
+		.pc_decode = nfssvc_decode_voidarg,
+		.pc_encode = nfssvc_encode_voidres,
+		.pc_argsize = sizeof(struct nfsd_voidargs),
+		.pc_ressize = sizeof(struct nfsd_voidres),
 		.pc_cachetype = RC_NOCACHE,
 		.pc_xdrressize = 1,
 	},