virtio_net: lower limit on buffer size

commit d85b758f ("virtio_net: fix support for small rings")
was supposed to increase the buffer size for small rings but had an
unintentional side effect of decreasing it for large rings. This seems
to break some setups - it's not yet clear why, but increasing buffer
size back to what it was before helps.

Fixes: d85b758f ("virtio_net: fix support for small rings")
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Reported-by: "J. Bruce Fields" <bfields@fieldses.org>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: "J. Bruce Fields" <bfields@fieldses.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>