selftests/bpf: Fix multiple segfaults in bpf_tcp_ca

Several test cases create a bpf_link from a struct_ops but attempt to use
it without first error-checking. This results is segfaults if the arch does
not yet support BPF struct_ops (e.g. mips64el):

    # ./test_progs -n 29/9
    test_update_ca:PASS:open 0 nsec
    test_update_ca:FAIL:attach_struct_ops unexpected error: -524
    do_test:PASS:socket 0 nsec
    do_test:PASS:socket 0 nsec
    settcpca:FAIL:setsockopt unexpected setsockopt: actual -1 == expected -1
    test_update_ca:FAIL:ca1_ca1_cnt unexpected ca1_ca1_cnt: actual 0 <= expected 0
    torvalds#29/9    bpf_tcp_ca/update_ca:FAIL
    torvalds#29      bpf_tcp_ca:FAIL
    Caught signal torvalds#11!
    Stack trace:
    <backtrace not supported>
    [17900.243360] do_page_fault(): sending SIGSEGV to test_progs for invalid read access from 0000000000000020
    [17900.243799] epc = 00000001204f817c in test_progs[4f817c,120000000+c53000]
    [17900.244209] ra  = 00000001204f8108 in test_progs[4f8108,120000000+c53000]
    Segmentation fault

Add the missing error handling to allow these tests to fail gracefully.

Fixes: 06da9f3 ("selftests/bpf: Test switching TCP Congestion Control algorithms.")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>

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

@@ -411,7 +411,10 @@ static void test_update_ca(void)
 		return;
 
 	link = bpf_map__attach_struct_ops(skel->maps.ca_update_1);
-	ASSERT_OK_PTR(link, "attach_struct_ops");
+	if (!ASSERT_OK_PTR(link, "attach_struct_ops")) {
+		tcp_ca_update__destroy(skel);
+		return;
+	}
 
 	do_test(&opts);
 	saved_ca1_cnt = skel->bss->ca1_cnt;
@@ -447,7 +450,10 @@ static void test_update_wrong(void)
 		return;
 
 	link = bpf_map__attach_struct_ops(skel->maps.ca_update_1);
-	ASSERT_OK_PTR(link, "attach_struct_ops");
+	if (!ASSERT_OK_PTR(link, "attach_struct_ops")) {
+		tcp_ca_update__destroy(skel);
+		return;
+	}
 
 	do_test(&opts);
 	saved_ca1_cnt = skel->bss->ca1_cnt;
@@ -473,25 +479,27 @@ static void test_mixed_links(void)
 		.cb_opts	= &cb_opts,
 	};
 	struct tcp_ca_update *skel;
-	struct bpf_link *link, *link_nl;
+	struct bpf_link *link = NULL, *link_nl = NULL;
 	int err;
 
 	skel = tcp_ca_update__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "open"))
 		return;
 
 	link_nl = bpf_map__attach_struct_ops(skel->maps.ca_no_link);
-	ASSERT_OK_PTR(link_nl, "attach_struct_ops_nl");
+	if (!ASSERT_OK_PTR(link_nl, "attach_struct_ops_nl"))
+		goto done;
 
 	link = bpf_map__attach_struct_ops(skel->maps.ca_update_1);
-	ASSERT_OK_PTR(link, "attach_struct_ops");
+	if (!ASSERT_OK_PTR(link, "attach_struct_ops"))
+		goto done;
 
 	do_test(&opts);
 	ASSERT_GT(skel->bss->ca1_cnt, 0, "ca1_ca1_cnt");
 
 	err = bpf_link__update_map(link, skel->maps.ca_no_link);
 	ASSERT_ERR(err, "update_map");
-
+done:
 	bpf_link__destroy(link);
 	bpf_link__destroy(link_nl);
 	tcp_ca_update__destroy(skel);
@@ -536,7 +544,10 @@ static void test_link_replace(void)
 	bpf_link__destroy(link);
 
 	link = bpf_map__attach_struct_ops(skel->maps.ca_update_2);
-	ASSERT_OK_PTR(link, "attach_struct_ops_2nd");
+	if (!ASSERT_OK_PTR(link, "attach_struct_ops_2nd")) {
+		tcp_ca_update__destroy(skel);
+		return;
+	}
 
 	/* BPF_F_REPLACE with a wrong old map Fd. It should fail!
 	 *