selftests/bpf: S/iptables/iptables-legacy/ in the bpf_nf and xdp_synproxy test

The recent vm image in CI has reported error in selftests that use
the iptables command.  Manu Bretelle has pointed out the difference
in the recent vm image that the iptables is sym-linked to the iptables-nft.
With this knowledge,  I can also reproduce the CI error by manually running
with the 'iptables-nft'.

This patch is to replace the iptables command with iptables-legacy
to unblock the CI tests.
Signed-off-by: NMartin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NDavid Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20221012221235.3529719-1-martin.lau@linux.dev

tools/testing/selftests/bpf/prog_tests/bpf_nf.c

@@ -49,14 +49,14 @@ static int connect_to_server(int srv_fd)
 
 static void test_bpf_nf_ct(int mode)
 {
-	const char *iptables = "iptables -t raw %s PREROUTING -j CONNMARK --set-mark 42/0";
+	const char *iptables = "iptables-legacy -t raw %s PREROUTING -j CONNMARK --set-mark 42/0";
 	int srv_fd = -1, client_fd = -1, srv_client_fd = -1;
 	struct sockaddr_in peer_addr = {};
 	struct test_bpf_nf *skel;
 	int prog_fd, err;
 	socklen_t len;
 	u16 srv_port;
-	char cmd[64];
+	char cmd[128];
 	LIBBPF_OPTS(bpf_test_run_opts, topts,
 		.data_in = &pkt_v4,
 		.data_size_in = sizeof(pkt_v4),
@@ -69,7 +69,7 @@ static void test_bpf_nf_ct(int mode)
 
 	/* Enable connection tracking */
 	snprintf(cmd, sizeof(cmd), iptables, "-A");
-	if (!ASSERT_OK(system(cmd), "iptables"))
+	if (!ASSERT_OK(system(cmd), cmd))
 		goto end;
 
 	srv_port = (mode == TEST_XDP) ? 5005 : 5006;

tools/testing/selftests/bpf/prog_tests/xdp_synproxy.c

@@ -94,12 +94,12 @@ static void test_synproxy(bool xdp)
 	SYS("sysctl -w net.ipv4.tcp_syncookies=2");
 	SYS("sysctl -w net.ipv4.tcp_timestamps=1");
 	SYS("sysctl -w net.netfilter.nf_conntrack_tcp_loose=0");
-	SYS("iptables -t raw -I PREROUTING \
+	SYS("iptables-legacy -t raw -I PREROUTING \
 	    -i tmp1 -p tcp -m tcp --syn --dport 8080 -j CT --notrack");
-	SYS("iptables -t filter -A INPUT \
+	SYS("iptables-legacy -t filter -A INPUT \
 	    -i tmp1 -p tcp -m tcp --dport 8080 -m state --state INVALID,UNTRACKED \
 	    -j SYNPROXY --sack-perm --timestamp --wscale 7 --mss 1460");
-	SYS("iptables -t filter -A INPUT \
+	SYS("iptables-legacy -t filter -A INPUT \
 	    -i tmp1 -m state --state INVALID -j DROP");
 
 	ctrl_file = SYS_OUT("./xdp_synproxy --iface tmp1 --ports 8080 \