kernel - Allow rn_inithead() to be called early

kernel - Allow rn_inithead() to be called early

* Allow rn_inithead() to be called earlier than rn_init().  rn_init() is
  called very late and was responsible for creating the all-ones and
  all-zeros keys.  It also required the proto domains to be initialized(?).

* A PF module preload was calling rn_inithead() before the all-ones and
  all-zeros keys could be allocated, resulting in a crash.