selftests/bpf: Add cb_refs test to s390x deny list

The cb_refs BPF selftest is failing execution on s390x machines. This is
a newly added test that requires a feature not presently supported on
this architecture.

Denylist the test for this architecture.

Fixes: 3cf7e7d8685c ("selftests/bpf: Add tests for reference state fixes for callbacks")
Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220824163906.1186832-1-deso@posteo.netSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/DENYLIST.s390x

@@ -66,3 +66,4 @@ select_reuseport                         # intermittently fails on new s390x set
 xdp_synproxy                             # JIT does not support calling kernel function                                (kfunc)
 unpriv_bpf_disabled                      # fentry
 setget_sockopt                           # attach unexpected error: -524                                               (trampoline)
+cb_refs                                  # expected error message unexpected error: -524                               (trampoline)