virtio_net: free xdp shinfo frags when build_skb_from_xdp_buff() fails

build_skb_from_xdp_buff() may return NULL, in this case
we need to free the frags of xdp shinfo.

Fixes: fab89baf ("virtio-net: support multi-buffer xdp")
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>