[IPV4]: fib_trie: Use const

Use const where possible and get rid of EXTRACT() macro
that was never used.

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