qlen check in tun.c

[Jerry Chu <hkchu@google.com>]

In tun_net_xmit() the max qlen is computed as
dev->tx_queue_len / tun->numqueues. For multi-queue configuration the
latter may be way too small, forcing one to adjust txqueuelen based
on number of queues created. (Well the default txqueuelen of
500/TUN_READQ_SIZE already seems too small even for single queue.)

Wouldn't it be better to simply use dev->tx_queue_len to cap the qlen of
each queue? This also seems to be more consistent with h/w multi-queues.

Also is there any objection to increase MAX_TAP_QUEUES from 8 to 16?
Yes it will take up more space in struct tun_struct. But we are
hitting the perf limit of 8 queues.

Thanks,

Jerry