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.