net: ip_rt_send_redirect() optimization

While doing some forwarding benchmarks, I noticed
ip_rt_send_redirect() is rather expensive, even if send_redirects is
false for the device.

Fix is to avoid two atomic ops, we dont really need to take a
reference on in_dev

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>