virtio_net: xsk: rx: support recv small mode

In the process:
1. We may need to copy data to create skb for XDP_PASS.
2. We may need to call xsk_buff_free() to release the buffer.
3. The handle for xdp_buff is difference from the buffer.

If we pushed this logic into existing receive handle(merge and small),
we would have to maintain code scattered inside merge and small (and big).
So I think it is a good choice for us to put the xsk code into an
independent function.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20240708112537.96291-10-xuanzhuo@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>