ipv4: fix rcu lock imbalance in fib_select_default()

Commit 0c838ff1 (ipv4: Consolidate all default route selection
implementations.) forgot to remove one rcu_read_unlock() from
fib_select_default().
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_semantics.c

@@ -1189,7 +1189,7 @@ void fib_select_default(struct fib_result *res)
 		fib_result_assign(res, last_resort);
 	tb->tb_default = last_idx;
 out:
-	rcu_read_unlock();
+	return;
 }
 
 #ifdef CONFIG_IP_ROUTE_MULTIPATH