selftests/bpf: Don't hard-code root cgroup id

Commit 40430452 ("kernfs: use 64bit inos if ino_t is 64bit") changed
the way cgroup ids are exposed to the userspace. Instead of assuming
fixed root id, let's query it.

Fixes: 40430452 ("kernfs: use 64bit inos if ino_t is 64bit")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191202200143.250793-1-sdf@google.com