xen/netback: don't call kfree_skb() under spin_lock_irqsave()

It is not allowed to call kfree_skb() from hardware interrupt
context or with interrupts being disabled. So replace kfree_skb()
with dev_kfree_skb_irq() under spin_lock_irqsave().

Fixes: be81992f

 ("xen/netback: don't queue unlimited number of packages")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20221205141333.3974565-1-yangyingliang@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>