macvtap: correctly free skb during socket destruction

We should use kfree_skb() instead of kfree() to free an skb.

Fixes: 362899b8

 ("macvtap: switch to use skb array")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>