FIB: store the node type not the function pointer.

FIB: store the node type not the function pointer.

Saves memory at no appreciable performance cost.
before:
  DBGvpp# sh fib mem
  FIB memory
               Name               Size  in-use /allocated   totals
               Entry               80      7   /   150      560/12000

after:
  DBGvpp# sh fib mem
  FIB memory
               Name               Size  in-use /allocated   totals
               Entry               72      7   /    7       504/504

Change-Id: Ic5d3920ceb57b54260dc9af2078c26484335fef1
Signed-off-by: Neale Ranns <nranns@cisco.com>