selftests/bpf: track tcp payload offset as scalar in xdp_synproxy

selftests/bpf: track tcp payload offset as scalar in xdp_synproxy

This change prepares syncookie_{tc,xdp} for update in callbakcs
verification logic. To allow bpf_loop() verification converge when
multiple callback itreations are considered:
- track offset inside TCP payload explicitly, not as a part of the
  pointer;
- make sure that offset does not exceed MAX_PACKET_OFF enforced by
  verifier;
- make sure that offset is tracked as unbound scalar between
  iterations, otherwise verifier won't be able infer that bpf_loop
  callback reaches identical states.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>