tuntap: Increase the number of queues in tun.

Networking under kvm works best if we allocate a per-vCPU RX and TX
queue in a virtual NIC. This requires a per-vCPU queue on the host side.

It is now safe to increase the maximum number of queues.
Preceding patch: 'net: allow large number of rx queues'
made sure this won't cause failures due to high order memory
allocations. Increase it to 256: this is the max number of vCPUs
KVM supports.

Size of tun_struct changes from 8512 to 10496 after this patch. This keeps
pages allocated for tun_struct before and after the patch to 3.

Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Reviewed-by: David Gibson <dgibson@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>