NFSD: Encoder and decoder functions are always present

nfsd_dispatch() is a hot path. Let's optimize the XDR method calls
for the by-far common case, which is that the XDR methods are indeed
present.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c
index 63e5a48..578fc9f 100644
--- a/fs/nfsd/nfs4proc.c
+++ b/fs/nfsd/nfs4proc.c
@@ -3279,6 +3279,7 @@ 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),