net: make struct net_device::tx_queue_len unsigned int

net: make struct net_device::tx_queue_len unsigned int

4 billion packet queue is something unthinkable so use 32-bit value
for now.

Space savings on x86_64:

	add/remove: 0/0 grow/shrink: 3/70 up/down: 16/-131 (-115)
	function                                     old     new   delta
	change_tx_queue_len                           94     108     +14
	qdisc_create                                1176    1177      +1
	alloc_netdev_mqs                            1124    1125      +1
	xenvif_alloc                                 533     532      -1
	x25_asy_setup                                167     166      -1
			...
	tun_queue_resize                             945     940      -5
	pfifo_fast_enqueue                           167     162      -5
	qfq_init_qdisc                               168     158     -10
	tap_queue_resize                             810     799     -11
	transmit                                     719     698     -21

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>