selftests: bpf: fix cgroup_hierarchical_stats

selftests: bpf: fix cgroup_hierarchical_stats

The cgroup_hierarchical_stats selftests uses an fentry program attached
to cgroup_attach_task and then passes the received &dst_cgrp->self to
the css_rstat_updated() kfunc. The verifier now assumes that all kfuncs
only takes trusted pointer arguments, and pointers received by fentry
are not marked trustes by default.

Use a tp_btf program in place for fentry for this test, pointers
received by tp_btf programs are marked trusted by the verifier.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102180038.2708325-10-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>