macvlan: fix a race on port dismantle and possible skb leaks

We need to cancel the work queue after rcu grace period,
otherwise it can be rescheduled by incoming packets.

We need to purge queue if some skbs are still in it.

We can use __skb_queue_head_init() variant in
macvlan_process_broadcast()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 412ca155 ("macvlan: Move broadcasts into a work queue")
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>