selftests/bpf: Check stack_mprotect() return value

If stack_mprotect() succeeds, errno is not changed. This can produce
misleading error messages, that show stale errno.

Signed-off-by: Ilya Leoshkevich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	int prog_fd;
 	int lsm_fd = -1;
 	LIBBPF_OPTS(bpf_link_create_opts, link_opts);
+	int err;
 
 	skel->bss->lsm_res = 0;
 
···
 	if (!ASSERT_GE(lsm_fd, 0, "lsm.link_create"))
 		goto cleanup;
 
-	stack_mprotect();
-	if (!ASSERT_EQ(errno, EPERM, "stack_mprotect"))
+	err = stack_mprotect();
+	if (!ASSERT_EQ(err, -1, "stack_mprotect") ||
+	    !ASSERT_EQ(errno, EPERM, "stack_mprotect"))
 		goto cleanup;
 
 	usleep(1);

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

···
 	skel->bss->monitored_pid = getpid();
 
 	err = stack_mprotect();
-	if (!ASSERT_EQ(errno, EPERM, "stack_mprotect"))
+	if (!ASSERT_EQ(err, -1, "stack_mprotect") ||
+	    !ASSERT_EQ(errno, EPERM, "stack_mprotect"))
 		return err;
 
 	ASSERT_EQ(skel->bss->mprotect_count, 1, "mprotect_count");