go.net/ipv4: add datagram benchmark

Current result on darwin/amd64:
BenchmarkReadWriteNetUDP-2	  100000	     16112 ns/op	     330 B/op	       6 allocs/op
BenchmarkReadWriteIPv4UDP-2	  200000	     14337 ns/op	     654 B/op	      14 allocs/op

Update golang/go#3684

R=dave
CC=golang-dev
https://golang.org/cl/10125044
1 file changed