virtio_net: virtnet_build_xdp_buff_mrg() auto release xdp shinfo

virtnet_build_xdp_buff_mrg() auto release xdp shinfo then the caller no
need to careful the xdp shinfo.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>