macvtap: switch to use skb array

This patch switch to use skb array instead of sk_receive_queue to
avoid spinlock contentions. Tests shows about 21% improvements for
guest rx pps:

Before: 1472731 pkts/s
After:  1786289 pkts/s

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>