Add size flag to demo pinger

Adds a command line flag to the demo pinger which lets the user set the
the size of the ICMP payload. This is useful for testing. The default is
set at 16 which is the default for an unconfigured pinger.

Signed-off-by: Charlie Jonas <charlie@charliejonas.co.uk>

cmd/ping/ping.go

@@ -31,12 +31,16 @@ Examples:
 
     # Send a privileged raw ICMP ping
     sudo ping --privileged www.google.com
+
+    # Send ICMP messages with a 100-byte payload
+    ping -s 100 1.1.1.1
 `
 
 func main() {
 	timeout := flag.Duration("t", time.Second*100000, "")
 	interval := flag.Duration("i", time.Second, "")
 	count := flag.Int("c", -1, "")
+	size := flag.Int("s", 16, "")
 	privileged := flag.Bool("privileged", false, "")
 	flag.Usage = func() {
 		fmt.Print(usage)
@@ -81,6 +85,7 @@ func main() {
 	}
 
 	pinger.Count = *count
+	pinger.Size = *size
 	pinger.Interval = *interval
 	pinger.Timeout = *timeout
 	pinger.SetPrivileged(*privileged)