selftests: tc-testing: add act_ct test for malformed header handling

selftests: tc-testing: add act_ct test for malformed header handling

Add a tdc case covering the leak fixed by the previous patch.

The test attaches "action ct" to a clsact ingress chain and injects ten
IPv6 frames whose nexthdr says hop-by-hop but which carry nothing after
the 40-byte header, so ipv6_find_hdr() fails and
tcf_ct_ipv6_is_fragment() returns -EPROTO.

Before the fix act_ct returned TC_ACT_CONSUMED for these packets, so
tc_run() never reached its TC_ACT_SHOT arm and the clsact drop counter
stayed at zero while the skbs leaked. After the fix the packets are
dropped properly and the counter reflects them, which is what the test
matches on:

  before:  Sent 476 bytes 11 pkt (dropped 0, overlimits 0 requeues 0)
  after:   Sent 400 bytes 10 pkt (dropped 10, overlimits 0 requeues 0)

Signed-off-by: Hyunjung Ko <hj351016@gmail.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260806101235.809370-2-hj351016@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>