bpf: Implement mprog API on top of existing cgroup progs

bpf: Implement mprog API on top of existing cgroup progs

Current cgroup prog ordering is appending at attachment time. This is not
ideal. In some cases, users want specific ordering at a particular cgroup
level. To address this, the existing mprog API seems an ideal solution with
supporting BPF_F_BEFORE and BPF_F_AFTER flags.

But there are a few obstacles to directly use kernel mprog interface.
Currently cgroup bpf progs already support prog attach/detach/replace
and link-based attach/detach/replace. For example, in struct
bpf_prog_array_item, the cgroup_storage field needs to be together
with bpf prog. But the mprog API struct bpf_mprog_fp only has bpf_prog
as the member, which makes it difficult to use kernel mprog interface.

In another case, the current cgroup prog detach tries to use the
same flag as in attach. This is different from mprog kernel interface
which uses flags passed from user space.

So to avoid modifying existing behavior, I made the following changes to
support mprog API for cgroup progs:
 - The support is for prog list at cgroup level. Cross-level prog list
   (a.k.a. effective prog list) is not supported.
 - Previously, BPF_F_PREORDER is supported only for prog attach, now
   BPF_F_PREORDER is also supported by link-based attach.
 - For attach, BPF_F_BEFORE/BPF_F_AFTER/BPF_F_ID/BPF_F_LINK is supported
   similar to kernel mprog but with different implementation.
 - For detach and replace, use the existing implementation.
 - For attach, detach and replace, the revision for a particular prog
   list, associated with a particular attach type, will be updated
   by increasing count by 1.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250606163141.2428937-1-yonghong.song@linux.dev