i40e: Remove rx page reuse double count

Page reuse was being tracked from two locations:
  - i40e_reuse_rx_page (via 40e_clean_rx_irq), and
  - i40e_alloc_mapped_page

Remove the double count and only count reuse from i40e_alloc_mapped_page
when the page is about to be reused.

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>