virtio-net: free bufs correctly on invalid packet length

When a packet with invalid length arrives, ensure that the packet
is freed correctly if mergeable packet buffers and big packets
(GUEST_TSO4) are both enabled.

Signed-off-by: Michael Dalton <mwdalton@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>