commit d85b758f upstream.

When ring size is small (<32 entries) making buffers smaller means a
full ring might not be able to hold enough buffers to fit a single large
packet.

Make sure a ring full of buffers is large enough to allow at least one
packet of max size.

Fixes: 2613af0e ("virtio_net: migrate mergeable rx buffers to page frag allocators")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
[bwh: Backported to 3.16:
 - There's no net_device::max_mtu, so always set packet_len = IP_MAX_MTU
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>