selftests/bpf: Add __noinline variant of cls_redirect selftest

As one of the most complicated and close-to-real-world programs, cls_redirect
is a good candidate to exercise libbpf's logic of handling bpf2bpf calls. So
add variant with using explicit __noinline for majority of functions except
few most basic ones. If those few functions are inlined, verifier starts to
complain about program instruction limit of 1mln instructions being exceeded,
most probably due to instruction overhead of doing a sub-program call.
Convert user-space part of selftest to have to sub-tests: with and without
inlining.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Lorenz Bauer <lmb@cloudflare.com>
Link: https://lore.kernel.org/bpf/20200903203542.15944-15-andriin@fb.com