virtio_net: fix rx_drops stat for small pkts

We found the stat of rx drops for small pkts does not increment when
build_skb fail, it's not coherent with other mode's rx drops stat.

Signed-off-by: Wenliang Wang <wangwenliang.1995@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>