igb: Add mechanism for detecting latched hardware Rx timestamp

Add a check against possible Rx timestamp freezing in the hardware via
watchdog mechanism. This situation can occur when an Rx timestamp has been
latched, but the packet has been dropped because the Rx ring is full.

Whenever a packet comes in that should be timestamped, the Rx timestamp
gets latched into the hardware registers and we will store the jiffy value
in the rx_ring. The watchdog will keep track of his own jiffy timer
whenever there is no valid timestamp in the registers.

If the watchdog detects a valid timestamp in the registers, meaning that no
Rx packet has consumed it yet, it will check which time is most recent: the
last time in the watchdog or any time in the rx_rings. If the most recent
"event" was more than 5 seconds ago, it will flush the Rx timestamp and
print a warning message to the syslog.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Jacob Keller <Jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>