i40e: Add a stat tracking new RX page allocations

Add a counter for new page allocations in the i40e RX path. This stat is
accessible with ethtool.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Tested-by: Dave Switzer <david.switzer@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>