tools: bpftool: detect sub-programs from the eBPF sequence

This patch detect all sub-programs from the eBPF sequence and keep the
information in the new CFG data structure.

The detection algorithm is basically the same as the one in verifier except
we need to use insn->off instead of insn->imm to get the pc-relative call
offset. Because verifier has modified insn->off/insn->imm during finishing
the verification.

Also, we don't need to do some sanity checks as verifier has done them.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>