Merge branch 'fix-effective-prog-array-indexing-with-bpf_f_preorder'

Amery Hung says:

====================
Fix effective prog array indexing with BPF_F_PREORDER

This patchset fixes a cgroup effective-array indexing bug where
replace_effective_prog() and purge_effective_progs() used a linear hlist
position that doesn't match the array layout when BPF_F_PREORDER
programs are present, corrupting the array on link update and risking a
use-after-free in the detach fallback. It computes the slot via a shared
effective_prog_pos() helper and adds a cgroup_preorder selftest.

Changelog

v1 -> v2:
 - Also fix purge_effective_progs(), in addition to
   replace_effective_prog() (Sashiko).
 - selftest: Set err on bpf_link_create() failure so the failure is
   reported to the caller (Sashiko).
====================

Link: https://patch.msgid.link/20260619063520.2690547-1-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>