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().

src/core/bpf-firewall.c

@@ -547,9 +547,9 @@ int bpf_firewall_compile(Unit *u) {
         if (!cc)
                 return -EINVAL;
 
-        crt = unit_setup_cgroup_runtime(u);
+        crt = unit_get_cgroup_runtime(u);
         if (!crt)
-                return -ENOMEM;
+                return -ESTALE;
 
         if (bpf_program_supported() <= 0)
                 return log_unit_debug_errno(u, SYNTHETIC_ERRNO(EOPNOTSUPP),