selftests/bpf: Test cgroup link replace with BPF_F_PREORDER

selftests/bpf: Test cgroup link replace with BPF_F_PREORDER

Add a cgroup_preorder case that attaches a normal and a BPF_F_PREORDER
program to a cgroup (effective order [2, 1]), then replaces the normal
link's program via bpf_link_update() and checks the effective order
becomes [2, 3] — i.e. only the non-preorder slot changes. Without the
replace_effective_prog() fix the array is corrupted and the order is
wrong.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260619063520.2690547-3-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>