cgroup: make cgroup_bpf_prog_attach work when cgroup2 is not mounted

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7DNAP


CVE: N/A

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BPF_PROG_TYPE_CGROUP* bpf programs is associated with cgroup2. If cgroup2
is not mounted, the bpf program is associated with cgrp_dfl_root.cgrp
by default.

Then we can use it like below:
bpftool cgroup attach /sys/fs/cgroup/cpu sock_ops pinned /sys/fs/bpf/xxx

Signed-off-by: Liu Jian <liujian56@huawei.com>