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>

Author: Yonghong Song

tools/testing/selftests/bpf/progs/btf_type_tag_percpu.c

@@ -57,15 +57,15 @@ int BPF_PROG(test_percpu_load, struct cgroup *cgrp, const char *path)
 SEC("tp_btf/cgroup_mkdir")
 int BPF_PROG(test_percpu_helper, struct cgroup *cgrp, const char *path)
 {
-	struct cgroup_rstat_cpu *rstat;
+	struct css_rstat_cpu *rstat;
 	__u32 cpu;
 
 	cpu = bpf_get_smp_processor_id();
-	rstat = (struct cgroup_rstat_cpu *)bpf_per_cpu_ptr(
+	rstat = (struct css_rstat_cpu *)bpf_per_cpu_ptr(
 			cgrp->self.rstat_cpu, cpu);
 	if (rstat) {
 		/* READ_ONCE */
-		*(volatile int *)rstat;
+		*(volatile long *)rstat;
 	}
 
 	return 0;