selftest/bpf: Add test for bpf_getsockopt()

This patch removes the __bpf_getsockopt() which directly
reads the sk by using PTR_TO_BTF_ID.  Instead, the test now directly
uses the kernel bpf helper bpf_getsockopt() which supports all
the required optname now.

TCP_SAVE[D]_SYN and TCP_MAXSEG are not tested in a loop for all
the hooks and sock_ops's cb.  TCP_SAVE[D]_SYN only works
in passive connection.  TCP_MAXSEG only works when
it is setsockopt before the connection is established and
the getsockopt return value can only be tested after
the connection is established.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20220902002937.2896904-1-kafai@fb.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>