selftests/bpf: Fix bpf_nf selftest failure

For systems with missing iptables-legacy tool this selftest fails.

Add check to find if iptables-legacy tool is available and skip the
test if the tool is missing.

Fixes: de9c8d848d90 ("selftests/bpf: S/iptables/iptables-legacy/ in the bpf_nf and xdp_synproxy test")
Signed-off-by: Saket Kumar Bhaskar <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 		.repeat = 1,
 	);
 
+	if (SYS_NOFAIL("iptables-legacy --version")) {
+		fprintf(stdout, "Missing required iptables-legacy tool\n");
+		test__skip();
+		return;
+	}
+
 	skel = test_bpf_nf__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "test_bpf_nf__open_and_load"))
 		return;