ixgbevf: provide 64 bit statistics

Compute statistics per ring using 64 bits, and provide
network device stats in 64 bits.

It should make this driver multiqueue operations faster (no more cache
line ping pongs on netdev->stats structure)

Use u64_stats_sync infrastructure so that its safe on 32bit arches as
well.

Based on a prior patch from Stephen Hemminger

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>