MFP r287070,r287073: split radix implementation and route table structure.

MFP r287070,r287073: split radix implementation and route table structure.

There are number of radix consumers in kernel land (pf,ipfw,nfs,route)
  with different requirements. In fact, first 3 don't have _any_ requirements
  and first 2 does not use radix locking. On the other hand, routing
  structure do have these requirements (rnh_gen, multipath, custom
  to-be-added control plane functions, different locking).
Additionally, radix should not known anything about its consumers internals.

So, radix code now uses tiny 'struct radix_head' structure along with
  internal 'struct radix_mask_head' instead of 'struct radix_node_head'.
  Existing consumers still uses the same 'struct radix_node_head' with
  slight modifications: they need to pass pointer to (embedded)
  'struct radix_head' to all radix callbacks.

Routing code now uses new 'struct rib_head' with different locking macro:
  RADIX_NODE_HEAD prefix was renamed to RIB_ (which stands for routing
  information base).

New net/route_var.h header was added to hold routing subsystem internal
  data. 'struct rib_head' was placed there. 'struct rtentry' will also
  be moved there soon.