NFS: Add a private local dispatcher for NFSv4 callback operations

The client's NFSv4 callback service is the only remaining user of
svc_generic_dispatch().

Note that the NFSv4 callback service doesn't use the .pc_encode and
.pc_decode callouts in any substantial way, so they are removed.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/callback_xdr.c

@@ -992,6 +992,15 @@ static __be32 nfs4_callback_compound(struct svc_rqst *rqstp)
 	return rpc_success;
 }
 
+static int
+nfs_callback_dispatch(struct svc_rqst *rqstp, __be32 *statp)
+{
+	const struct svc_procedure *procp = rqstp->rq_procinfo;
+
+	*statp = procp->pc_func(rqstp);
+	return 1;
+}
+
 /*
  * Define NFS4 callback COMPOUND ops.
  */
@@ -1080,7 +1089,7 @@ const struct svc_version nfs4_callback_version1 = {
 	.vs_proc = nfs4_callback_procedures1,
 	.vs_count = nfs4_callback_count1,
 	.vs_xdrsize = NFS4_CALLBACK_XDRSIZE,
-	.vs_dispatch = NULL,
+	.vs_dispatch = nfs_callback_dispatch,
 	.vs_hidden = true,
 	.vs_need_cong_ctrl = true,
 };
@@ -1092,7 +1101,7 @@ const struct svc_version nfs4_callback_version4 = {
 	.vs_proc = nfs4_callback_procedures1,
 	.vs_count = nfs4_callback_count4,
 	.vs_xdrsize = NFS4_CALLBACK_XDRSIZE,
-	.vs_dispatch = NULL,
+	.vs_dispatch = nfs_callback_dispatch,
 	.vs_hidden = true,
 	.vs_need_cong_ctrl = true,
 };