[IPV4]: FIB Trie cleanups.

Below is a patch that cleans up some of this, supposedly without
changing any behaviour:

* Whitespace cleanups
* Introduce DBG()
* BUG_ON() instead of if () { BUG(); }
* Remove some of the deep nesting to make the code flow more
  comprehensible
* Some mask operations were simplified

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>