ipv4: Remove leftover rcu_read_unlock calls from __mkroute_output()

Commit "fib: RCU conversion of fib_lookup()" removed rcu_read_lock() from
__mkroute_output but left a couple of calls to rcu_read_unlock() in there.
This causes lockdep to complain that the rcu_read_unlock() call in
__ip_route_output_key causes a lock inbalance and quickly crashes the
kernel. The below fixes this for me.
Signed-off-by: NDimitris Michailidis <dm@chelsio.com>
Acked-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2396,12 +2396,10 @@ static int __mkroute_output(struct rtable **result,
 
 
 	rth = dst_alloc(&ipv4_dst_ops);
-	if (!rth) {
-		rcu_read_unlock();
+	if (!rth)
 		return -ENOBUFS;
-	}
+
 	in_dev_hold(in_dev);
-	rcu_read_unlock();
 	rth->idev = in_dev;
 
 	atomic_set(&rth->dst.__refcnt, 1);