[IPV4] fib_trie: style cleanup

Style cleanups:
      * make check_leaf return -1 or plen, rather than by reference
      * Get rid of #ifdef that is always set
      * split out embedded function calls in if statements.
      * checkpatch warnings

Signed-off-by: Stephen Hemminger <stephen.hemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>