xen-netfront: use skb_partial_csum_set() to simplify the codes

use skb_partial_csum_set() to simplify the codes

Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>