core/bpf-firewall: replace unnecessary unit_setup_cgroup_runtime() with unit_get_cgro...

core/bpf-firewall: replace unnecessary unit_setup_cgroup_runtime() with unit_get_cgroup_runtime()

Except for the test, bpf_firewall_compile() is only called by the following:
  cgroup_context_apply() -> cgroup_apply_firewall() -> bpf_firewall_compile()
and in the early stage of cgroup_context_apply(), it checks if the cgroup
runtime exists. Hence, it is not necessary to try to allocate the
runtime in bpf_firewall_compile().