selftests/bpf: DENYLIST.aarch64: Skip fexit_sleep again

Revert commit 90dc9460 ("selftests/bpf: DENYLIST.aarch64: Remove
fexit_sleep") again. The fix in 19d3c179 ("bpf, arm64: Fix trampoline
for BPF_TRAMP_F_CALL_ORIG") does not address all of the issues and BPF
CI is still hanging and timing out:

   https://github.com/kernel-patches/bpf/actions/runs/9905842936/job/27366435436

   [...]
   #89/11   fexit_bpf2bpf/func_replace_global_func:OK
   #89/12   fexit_bpf2bpf/fentry_to_cgroup_bpf:OK
   #89/13   fexit_bpf2bpf/func_replace_progmap:OK
   #89      fexit_bpf2bpf:OK
   Error: The operation was canceled.

Thus more investigation work & fixing is needed before the test can be put
in place again.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/bpf/20240705145009.32340-1-puranjay@kernel.org

tools/testing/selftests/bpf/DENYLIST.aarch64

 bpf_cookie/multi_kprobe_attach_api               # kprobe_multi_link_api_subtest:FAIL:fentry_raw_skel_load unexpected error: -3
 bpf_cookie/multi_kprobe_link_api                 # kprobe_multi_link_api_subtest:FAIL:fentry_raw_skel_load unexpected error: -3
+fexit_sleep                                      # The test never returns. The remaining tests cannot start.
 kprobe_multi_bench_attach                        # needs CONFIG_FPROBE
 kprobe_multi_test                                # needs CONFIG_FPROBE
 module_attach                                    # prog 'kprobe_multi': failed to auto-attach: -95