cgroup/bpf: only cgroup v2 can be attached by bpf programs

[ Upstream commit 2190df6c ]

Only cgroup v2 can be attached by bpf programs, so this patch introduces
that cgroup_bpf_inherit and cgroup_bpf_offline can only be called in
cgroup v2, and this can fix the memleak mentioned by commit 04f8ef56
("cgroup: Fix memory leak caused by missing cgroup_bpf_offline"), which
has been reverted.

Fixes: 2b0d3d3e ("percpu_ref: reduce memory footprint of percpu_ref in fast path")
Fixes: 4bfc0bb2 ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")
Link: https://lore.kernel.org/cgroups/aka2hk5jsel5zomucpwlxsej6iwnfw4qu5jkrmjhyfhesjlfdw@46zxhg5bdnr7/


Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>