selftests/bpf: add wrong type test to cgroup dev

Current cgroup_dev test mostly tests that device operation is accepted or
refused base on passed major/minor (and so, any operation performed during
test involves only char device)

Add a small subtest ensuring that the device type passed to bpf program
allows it to take decisions as well.

Reviewed-by: Alan Maguire <[e-mail]>
Acked-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

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

···
 	if (test__start_subtest("deny-write"))
 		test_write("/dev/zero", buf, TEST_BUFFER_SIZE, -1, EPERM);
 
+	if (test__start_subtest("deny-mknod-wrong-type"))
+		test_mknod("/dev/test_dev_cgroup_block", S_IFBLK, 1, 3, -1,
+			   EPERM);
+
 cleanup_progs:
 	dev_cgroup__destroy(skel);
 cleanup_cgroup: