samples/bpf: Convert magic numbers to names in multi-prog cgroup test case

Attach flag 1 == BPF_F_ALLOW_OVERRIDE; attach flag 2 == BPF_F_ALLOW_MULTI.
Update the calls to bpf_prog_attach() in test_cgrp2_attach2.c to use the
names over the magic numbers.

Fixes: 39323e78

 ("samples/bpf: add multi-prog cgroup test case")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>