net: add netdev_for_each_upper_dev_rcu()

The new macro netdev_for_each_upper_dev_rcu(dev, upper, iter) iterates
through the dev->upper_dev_list starting from the first element, using
the netdev_upper_get_next_dev_rcu(dev, &iter).

Must be called under RCU read lock.

CC: "David S. Miller" <davem@davemloft.net>
CC: Eric Dumazet <edumazet@google.com>
CC: Jiri Pirko <jiri@resnulli.us>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
CC: Cong Wang <amwang@redhat.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>