libbpf: Fix up verifier log for unguarded failed CO-RE relos

Teach libbpf to post-process BPF verifier log on BPF program load
failure and detect known error patterns to provide user with more
context.

Currently there is one such common situation: an "unguarded" failed BPF
CO-RE relocation. While failing CO-RE relocation is expected, it is
expected to be property guarded in BPF code such that BPF verifier
always eliminates BPF instructions corresponding to such failed CO-RE
relos as dead code. In cases when user failed to take such precautions,
BPF verifier provides the best log it can:

  123: (85) call unknown#195896080
  invalid func unknown#195896080

Such incomprehensible log error is due to libbpf "poisoning" BPF
instruction that corresponds to failed CO-RE relocation by replacing it
with invalid `call 0xbad2310` instruction (195896080 == 0xbad2310 reads
"bad relo" if you squint hard enough).

Luckily, libbpf has all the necessary information to look up CO-RE
relocation that failed and provide more human-readable description of
what's going on:

  5: <invalid CO-RE relocation>
  failed to resolve CO-RE relocation <byte_off> [6] struct task_struct___bad.fake_field_subprog (0:2 @ offset 8)

This hopefully makes it much easier to understand what's wrong with
user's BPF program without googling magic constants.

This BPF verifier log fixup is setup to be extensible and is going to be
used for at least one other upcoming feature of libbpf in follow up patches.
Libbpf is parsing lines of BPF verifier log starting from the very end.
Currently it processes up to 10 lines of code looking for familiar
patterns. This avoids wasting lots of CPU processing huge verifier logs
(especially for log_level=2 verbosity level). Actual verification error
should normally be found in last few lines, so this should work
reliably.

If libbpf needs to expand log beyond available log_buf_size, it
truncates the end of the verifier log. Given verifier log normally ends
with something like:

  processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

... truncating this on program load error isn't too bad (end user can
always increase log size, if it needs to get complete log).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220426004511.2691730-10-andrii@kernel.org