selftests/bpf: fix lots of silly mistakes pointed out by compiler

Once we enable -Wall for BPF sources, compiler will complain about lots
of unused variables, variables that are set but never read, etc.

Fix all these issues first before enabling -Wall in Makefile.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230309054015.4068562-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add selftests for new cgroup local storage

Add four tests for new cgroup local storage, (1) testing bpf program helpers
and user space map APIs, (2) testing recursive fentry triggering won't deadlock,
(3) testing progs attached to cgroups, and (4) a negative test if the
bpf_cgrp_storage_get() helper key is not a cgroup btf id.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221026042911.675546-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>