selftests/bpf: Add test for redirect from qdisc qevent block

selftests/bpf: Add test for redirect from qdisc qevent block

Add a regression test for the NULL current->bpf_net_context deref hit
when a BPF classifier attached to a qdisc qevent block asks for a
redirect. The classifier runs from tcf_qevent_handle() on the qdisc
enqueue path, outside any bpf_net_context.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t qevent
  [...]
  + /etc/rcS.d/S50-startup
  ./test_progs -t qevent
  #496/1   tc_qevent/redirect_verdict:OK
  #496/2   tc_qevent/redirect_helper:OK
  #496     tc_qevent:OK
  Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260706185609.330006-4-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>