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/[e-mail]/

Cc: Ihor Solodrai <[e-mail]>
Cc: JP Kobryn <[e-mail]>
Signed-off-by: Yonghong Song <[e-mail]>
Acked-by: JP Kobryn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 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;