routing: limit routing info size during pathfinding

Also the max hop count check can be removed, because the real bound is
the payload size. By moving the check inside the search loop, we now
also backtrack when we hit the limit.

routing/pathfind_test.go

@@ -1393,8 +1393,19 @@ func TestNewRoutePathTooLong(t *testing.T) {
 	// Assert that finding a 21 hop route fails.
 	node21 := ctx.keyFromAlias("node-21")
 	_, err = ctx.findPath(node21, payAmt)
-	if err != errMaxHopsExceeded {
-		t.Fatalf("expected route too long, but got %v", err)
+	if err != errNoPathFound {
+		t.Fatalf("not route error expected, but got %v", err)
+	}
+
+	// Assert that we can't find a 20 hop route if custom records make it
+	// exceed the maximum payload size.
+	ctx.restrictParams.DestFeatures = tlvFeatures
+	ctx.restrictParams.DestCustomRecords = map[uint64][]byte{
+		100000: bytes.Repeat([]byte{1}, 100),
+	}
+	_, err = ctx.findPath(node20, payAmt)
+	if err != errNoPathFound {
+		t.Fatalf("not route error expected, but got %v", err)
 	}
 }