virtio_net: add checking sq is full inside xdp xmit

If the queue of xdp xmit is not an independent queue, then when the xdp
xmit used all the desc, the xmit from the __dev_queue_xmit() may encounter
the following error.

net ens4: Unexpected TXQ (0) queue failure: -28

This patch adds a check whether sq is full in xdp xmit.

Fixes: 56434a01 ("virtio_net: add XDP_TX support")
Reported-by: Yichun Zhang <yichun@openresty.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>