selftests/bpf: Switch SEC("classifier*") usage to a strict SEC("tc")

Convert all SEC("classifier*") uses to a new and strict SEC("tc")
section name. In reference_tracking selftests switch from ambiguous
searching by program title (section name) to non-ambiguous searching by
name in some selftests, getting closer to completely removing
bpf_object__find_program_by_title().

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210928161946.2512801-4-andrii@kernel.org

selftests: bpf: Test sockmap update from BPF

Add a test which copies a socket from a sockmap into another sockmap
or sockhash. This excercises bpf_map_update_elem support from BPF
context. Compare the socket cookies from source and destination to
ensure that the copy succeeded.

Also check that the verifier rejects map_update from unsafe contexts.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200821102948.21918-7-lmb@cloudflare.com