Fix kernel BUG on early access to /proc/tempesta/perfstat before APM is initialized (#2424)

[WitalyAnisimov]

Tempesta crashes with a kernel BUG when /proc/tempesta/perfstat is accessed early, before the cfgend() stage completes. This happens because tfw_apm_global_data is initialized too late — only in tfw_apm_cfgend().

issue#2424