selftests/bpf: Add tests for a store on a fault prone qdisc pointer

Cover the store which used to be left as a plain BPF_STX without an
exception table entry:

  1: R1=trusted_ptr_Qdisc()
  ; struct Qdisc *next = sch->next_sched;
  1: (79) r1 = *(u64 *)(r1 +216)        ; R1=ptr_Qdisc()
  ; next->limit = 1000;
  3: (63) *(u32 *)(r1 +20) = r2         ; R1=ptr_Qdisc() R2=1000

Assert that it is rejected now.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t ns_bpf_qdisc
  [...]
  #257/1   ns_bpf_qdisc/fifo:OK
  #257/2   ns_bpf_qdisc/fq:OK
  #257/3   ns_bpf_qdisc/attach to mq:OK
  #257/4   ns_bpf_qdisc/attach to non root:OK
  #257/5   ns_bpf_qdisc/incompl_ops:OK
  #257/6   ns_bpf_qdisc/invalid_dynptr:OK
  #257/7   ns_bpf_qdisc/invalid_dynptr_cross_frame:OK
  #257/8   ns_bpf_qdisc/invalid_dynptr_slice:OK
  #257/9   ns_bpf_qdisc/untrusted_write:OK
  #257/10  ns_bpf_qdisc/dynptr_use_after_invalidate_clone:OK
  #257     ns_bpf_qdisc:OK
  Summary: 1/10 PASSED, 0 SKIPPED, 0/0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260817141015.878071-3-daniel@iogearbox.net
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>