Add selftests for bpf_skb_ancestor_cgroup_id helper.

test_skb_cgroup_id.sh prepares testing interface and adds tc qdisc and
filter for it using BPF object compiled from test_skb_cgroup_id_kern.c
program.

BPF program in test_skb_cgroup_id_kern.c gets ancestor cgroup id using
the new helper at different levels of cgroup hierarchy that skb belongs
to, including root level and non-existing level, and saves it to the map
where the key is the level of corresponding cgroup and the value is its
id.

To trigger BPF program, user space program test_skb_cgroup_id_user is
run. It adds itself into testing cgroup and sends UDP datagram to
link-local multicast address of testing interface. Then it reads cgroup
ids saved in kernel for different levels from the BPF map and compares
them with those in user space. They must be equal for every level of
ancestry.

Example of run:
  # ./test_skb_cgroup_id.sh
  Wait for testing link-local IP to become available ... OK
  Note: 8 bytes struct bpf_elf_map fixup performed due to size mismatch!
  [PASS]
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>