virtio_net: check virtqueue_add_sgs() return value

As virtqueue_add_sgs() can fail, we should check the return value.

Addresses-Coverity-ID: 1464439 ("Unchecked return value")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>