igb: move alloc_failed and csum_err stats into per rx-ring stat

The allocation failed and checksum error stats are currently kept as a
global stat.  If we end up allocating the queues to multiple netdevs then
the global counter doesn't make much sense.  For this reason I felt it
necessary to move the alloc_rx_buff_failed stat into the rx_stats
portion of the rx_ring.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>