Implement UDP socket diagnostics

Signed-off-by: Sven Rebhan <srebhan@influxdata.com>

socket_test.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package netlink
@@ -75,3 +76,18 @@ func TestSocketDiagTCPInfo(t *testing.T) {
 		}
 	}
 }
+
+func TestSocketDiagUDPnfo(t *testing.T) {
+	for _, want := range []uint8{syscall.AF_INET, syscall.AF_INET6} {
+		result, err := SocketDiagUDPInfo(want)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		for _, r := range result {
+			if got := r.InetDiagMsg.Family; got != want {
+				t.Fatalf("protocol family = %v, want %v", got, want)
+			}
+		}
+	}
+}