[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: NJoonwoo Park <joonwpark81@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1214,6 +1214,9 @@ static int fn_trie_insert(struct fib_table *tb, struct fib_config *cfg)
 			struct fib_info *fi_drop;
 			u8 state;
 
+			if (fi->fib_treeref > 1)
+				goto out;
+
 			err = -ENOBUFS;
 			new_fa = kmem_cache_alloc(fn_alias_kmem, GFP_KERNEL);
 			if (new_fa == NULL)