cgroup: Add cgroup_get_from_fd

cgroup: Add cgroup_get_from_fd

Add a helper function to get a cgroup2 from a fd.  It will be
stored in a bpf array (BPF_MAP_TYPE_CGROUP_ARRAY) which will
be introduced in the later patch.

Signed-off-by: Martin KaFai Lau <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Tejun Heo <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Signed-off-by: David S. Miller <[email protected]>