__in_dev_get_rtnl() can use rtnl_dereference()

If caller holds RTNL, we dont need a memory barrier
(smp_read_barrier_depends) included in rcu_dereference().

Just use rtnl_dereference() to properly document the assertions.

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