net/packet: record allocations in benchmark.

net/packet/packet_test.go

@@ -266,6 +266,7 @@ func BenchmarkDecode(b *testing.B) {
 
 	for _, bench := range benches {
 		b.Run(bench.name, func(b *testing.B) {
+			b.ReportAllocs()
 			for i := 0; i < b.N; i++ {
 				var p Parsed
 				p.Decode(bench.buf)