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: Mix legacy (maps) and modern (vars) BPF in one test

Add selftest that combines two BPF programs within single BPF object
file such that one of the programs is using global variables, but can be
skipped at runtime on old kernels that don't support global data.
Another BPF program is written with the goal to be runnable on very old
kernels and only relies on explicitly accessed BPF maps.

Such test, run against old kernels (e.g., libbpf CI will run it against 4.9
kernel that doesn't support global data), allows to test the approach
and ensure that libbpf doesn't make unnecessary assumption about
necessary kernel features.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211123200105.387855-2-andrii@kernel.org