samples: bpf: force IPv4 in ping

ping localhost may default of IPv6 on modern systems, but
samples are trying to only parse IPv4. Force IPv4.

samples/bpf/tracex1_user.c doesn't interpret the packet so
we don't care which IP version will be used there.

Signed-off-by: Jakub Kicinski <[e-mail]>
Reviewed-by: Quentin Monnet <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

samples/bpf/sock_example.c

···
 {
 	FILE *f;
 
-	f = popen("ping -c5 localhost", "r");
+	f = popen("ping -4 -c5 localhost", "r");
 	(void)f;
 
 	return test_sock();

samples/bpf/sockex1_user.c

···
 	assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, prog_fd,
 			  sizeof(prog_fd[0])) == 0);
 
-	f = popen("ping -c5 localhost", "r");
+	f = popen("ping -4 -c5 localhost", "r");
 	(void) f;
 
 	for (i = 0; i < 5; i++) {

samples/bpf/sockex2_user.c

···
 	assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, prog_fd,
 			  sizeof(prog_fd[0])) == 0);
 
-	f = popen("ping -c5 localhost", "r");
+	f = popen("ping -4 -c5 localhost", "r");
 	(void) f;
 
 	for (i = 0; i < 5; i++) {

samples/bpf/sockex3_user.c

···
 			  sizeof(__u32)) == 0);
 
 	if (argc > 1)
-		f = popen("ping -c5 localhost", "r");
+		f = popen("ping -4 -c5 localhost", "r");
 	else
 		f = popen("netperf -l 4 localhost", "r");
 	(void) f;

samples/bpf/tracex2_user.c

···
 	signal(SIGTERM, int_exit);
 
 	/* start 'ping' in the background to have some kfree_skb events */
-	f = popen("ping -c5 localhost", "r");
+	f = popen("ping -4 -c5 localhost", "r");
 	(void) f;
 
 	/* start 'dd' in the background to have plenty of 'write' syscalls */