virtio_net: Remove BUG() to avoid machine dead

We should not directly BUG() when there is hdr error, it is
better to output a print when such error happens. Currently,
the caller of xmit_skb() already did it.

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>