selftests/bpf: Add selftest deny_namespace to s390x deny list

BPF CI reported that selftest deny_namespace failed with s390x.

  test_unpriv_userns_create_no_bpf:PASS:no-bpf unpriv new user ns 0 nsec
  test_deny_namespace:PASS:skel load 0 nsec
  libbpf: prog 'test_userns_create': failed to attach: ERROR: strerror_r(-524)=22
  libbpf: prog 'test_userns_create': failed to auto-attach: -524
  test_deny_namespace:FAIL:attach unexpected error: -524 (errno 524)
  #57/1    deny_namespace/unpriv_userns_create_no_bpf:FAIL
  #57      deny_namespace:FAIL

BPF program test_userns_create is a BPF LSM type program which is
based on trampoline and s390x does not support s390x. Let add the
test to x390x deny list to avoid this failure in BPF CI.

Signed-off-by: Yonghong Song <yhs@fb.com>

tools/testing/selftests/bpf/DENYLIST.s390x

@@ -75,3 +75,4 @@ user_ringbuf                             # failed to find kernel BTF type ID of
 lookup_key                               # JIT does not support calling kernel function                                (kfunc)
 verify_pkcs7_sig                         # JIT does not support calling kernel function                                (kfunc)
 kfunc_dynptr_param                       # JIT does not support calling kernel function                                (kfunc)
+deny_namespace                           # failed to attach: ERROR: strerror_r(-524)=22                                (trampoline)