selftests/bpf: update tcp_custom_syncookie to use scalar packet offset

This commit updates tcp_custom_syncookie.c:tcp_parse_option() to use
explicit packet offset (ctx->off) for packet access instead of ever
moving pointer (ctx->ptr), this reduces verification complexity:
- the tcp_parse_option() is passed as a callback to bpf_loop();
- suppose a checkpoint is created each time at function entry;
- the ctx->ptr is tracked by verifier as PTR_TO_PACKET;
- the ctx->ptr is incremented in tcp_parse_option(),
  thus umax_value field tracked for it is incremented as well;
- on each next iteration of tcp_parse_option()
  checkpoint from a previous iteration can't be reused
  for state pruning, because PTR_TO_PACKET registers are
  considered equivalent only if old->umax_value >= cur->umax_value;
- on the other hand, the ctx->off is a SCALAR,
  subject to widen_imprecise_scalars();
- it's exact bounds are eventually forgotten and it is tracked as
  unknown scalar at entry to tcp_parse_option();
- hence checkpoints created at the start of the function eventually
  converge.

The change is similar to one applied in [0] to xdp_synproxy_kern.c.

Comparing before and after with veristat yields following results:

File                             Insns (A)  Insns (B)  Insns      (DIFF)
-------------------------------  ---------  ---------  -----------------
test_tcp_custom_syncookie.bpf.o     466657      12423  -454234 (-97.34%)

[0] commit 977bc146 ("selftests/bpf: track tcp payload offset as scalar in xdp_synproxy")

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240222150300.14909-2-eddyz87@gmail.com


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