[IPV4]: FIB trie cleanup

This is a redo of earlier cleanup stuff:
	* replace DBG() macro with pr_debug()
	* get rid of duplicate extern's that are already in fib_lookup.h
	* use BUG_ON and WARN_ON
	* don't use BUG checks for null pointers where next statement would
	  get a fault anyway
	* remove debug printout when rebalance causes deep tree
	* remove trailing blanks

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>