i40e: convert queue stats to i40e_stats array

Use an i40e_stats array to handle the queue stats, instead of coding
similar functionality separately. Because of how the queue stats are
accessed on some kernels, we can't easily use i40e_add_ethtool_stats.

Instead, implement a separate helper, i40e_add_queue_stats, which we'll
use instead. This helper will correctly implement the
u64_stats_fetch_begin_irq logic and allow retries until successful. We
share the most complex code by re-using i40e_add_one_ethtool_stat.

This logic additionally easily supports skipping disabled rings by using
a ternary operator before calling the u64_stats_fetch_begin_irq()
function, so that we correctly zero-out the stats values without having
to perform two separate sections of code.

This significantly reduces the boiler plate code in
i40e_get_ethtool_stats, and helps keep the complex logic contained to as
few functions as possible.

With this patch, we've finally converted all the statistics to use the
helpers and the i40e_stats function.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>