selftests/bpf: Fix selftest btf_tag/btf_type_tag_percpu_vmlinux_helper failure

Ihor Solodrai reported selftest 'btf_tag/btf_type_tag_percpu_vmlinux_helper'
failure ([1]) during 6.16 merge window. The failure log:

  ...
  7: (15) if r0 == 0x0 goto pc+1        ; R0=ptr_css_rstat_cpu()
  ; *(volatile int *)rstat; @ btf_type_tag_percpu.c:68
  8: (61) r1 = *(u32 *)(r0 +0)
  cannot access ptr member updated_children with moff 0 in struct css_rstat_cpu with off 0 size 4

Two changes are needed. First, 'struct cgroup_rstat_cpu' needs to be
replaced with 'struct css_rstat_cpu' to be consistent with new data
structure. Second, layout of 'css_rstat_cpu' is changed compared
to 'cgroup_rstat_cpu'. The first member becomes a pointer so
the bpf prog needs to do 8-byte load instead of 4-byte load.

  [1] https://lore.kernel.org/bpf/6f688f2e-7d26-423a-9029-d1b1ef1c938a@linux.dev/



Cc: Ihor Solodrai <ihor.solodrai@linux.dev>
Cc: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: JP Kobryn <inwardvessel@gmail.com>
Link: https://lore.kernel.org/r/20250529201151.1787575-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>