macvlan: Use 'hash' iterators to simplify code

Use 'hash_for_each_rcu' and 'hash_for_each_safe' instead of hand writing
them. This saves some lines of code, reduce indentation and improve
readability.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>