selftests/bpf: Fix cgroup_mprog_ordering failure due to uninitialized variable

selftests/bpf: Fix cgroup_mprog_ordering failure due to uninitialized variable

On arm64, the cgroup_mprog_ordering selftest failed with test_progs run
when building with clang compiler. The reason is due to socklen_t optlen
not initialized.

In kernel function do_ip_getsockopt(), we have

        if (copy_from_sockptr(&len, optlen, sizeof(int)))
                return -EFAULT;
        if (len < 0)
                return -EINVAL;

The above 'len' variable is a negative value and hence the test failed.

But the test is okay on x86_64. I checked the x86_64 asm code and I didn't
see explicit initialization of 'optlen' but its value is 0 so kernel
didn't return error. This should be a pure luck.

Fix the bug by initializing 'oplen' var properly.

Fixes: e422d5f118e4 ("selftests/bpf: Add two selftests for mprog API based cgroup progs")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250611162103.1623692-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>