selftests/bpf: Add test for stale pkt range after scalar arithmetic

selftests/bpf: Add test for stale pkt range after scalar arithmetic

Extend the verifier_direct_packet_access BPF selftests to exercise the
verifier code paths which ensure that the pkt range is cleared after
add/sub alu with a known scalar. The tests reject the invalid access.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_direct
  [...]
  #592/35  verifier_direct_packet_access/direct packet access: pkt_range cleared after sub with known scalar:OK
  #592/36  verifier_direct_packet_access/direct packet access: pkt_range cleared after add with known scalar:OK
  #592/37  verifier_direct_packet_access/direct packet access: test3:OK
  #592/38  verifier_direct_packet_access/direct packet access: test3 @unpriv:OK
  #592/39  verifier_direct_packet_access/direct packet access: test34 (non-linear, cgroup_skb/ingress, too short eth):OK
  #592/40  verifier_direct_packet_access/direct packet access: test35 (non-linear, cgroup_skb/ingress, too short 1):OK
  #592/41  verifier_direct_packet_access/direct packet access: test36 (non-linear, cgroup_skb/ingress, long enough):OK
  #592     verifier_direct_packet_access:OK
  [...]
  Summary: 2/47 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260409155016.536608-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>