add benchmark filling a whole subnet

ipalloc/ipalloc_test.go

@@ -176,3 +176,12 @@ func TestInsert(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkFill(b *testing.B) {
+	root := &tree{prefix: netip.MustParsePrefix("0.0.0.0/20")}
+	for {
+		if alloc := root.Alloc(32); alloc == nil {
+			break
+		}
+	}
+}