net: new counter for tx_timeout errors in sysfs

This adds the /sys/class/net/DEV/queues/Q/tx_timeout attribute
containing the total number of timeout events on the given queue. It
is always available with CONFIG_SYSFS, independently of
CONFIG_RPS/XPS.

Credits to Stephen Hemminger for a preliminary version of this patch.

Tested:
  without CONFIG_SYSFS (compilation only)
  with sysfs and without CONFIG_RPS & CONFIG_XPS
  with sysfs and without CONFIG_RPS
  with sysfs and without CONFIG_XPS
  with defaults

Signed-off-by: David Decotigny <david.decotigny@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -246,6 +246,7 @@ static void dev_watchdog(unsigned long arg)
 				    time_after(jiffies, (trans_start +
 							 dev->watchdog_timeo))) {
 					some_queue_timedout = 1;
+					txq->trans_timeout++;
 					break;
 				}
 			}