[IPV4] fib_trie: fix duplicated route issue

http://bugzilla.kernel.org/show_bug.cgi?id=9493

The fib allows making identical routes with 'ip route replace'.
This patch makes the fib return -EEXIST if replacement would cause duplication.

Signed-off-by: Joonwoo Park <joonwpark81@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>