selftests/bpf: tc_tunnel - validate decap GSO and encapsulation state

tc_tunnel only partially validated decap state and missed some tunnel
cases. In particular, IPXIP decap checks were not exercised for
IPIP/SIT paths, and non-GSO decap encapsulation state was not
verified.

Tighten the test by:

- setting DECAP_IPXIP4/6 flags for IPIP/SIT/IP6 decap paths based on
  the outer tunnel header family;
- requiring needed DECAP enum values via CO-RE enum existence checks
  so missing kernel support fails fast;
- validating post-decap tunnel state for both GSO and non-GSO packets:
  expected gso_type bits must be cleared and skb->encapsulation must
  match remaining tunnel flags;
- removing forced TSO disable in the test harness so GSO validation is
  exercised.

This improves coverage for decap tunnel-state regressions and ensures
sit_none/ipip-style paths are checked correctly.

Signed-off-by: Nick Hudson <nhudson@akamai.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260812083115.73100-7-nhudson@akamai.com

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