i40e/i40evf: eliminate i40e_pull_tail()

Reorganize the i40e_pull_tail() logic, doing it in i40e_add_rx_frag()
where it's cheaper.  The igb driver does this the same way.

Also renames i40e_page_is_reserved() to reflect what it actually
tests.

Change-ID: Icd9cc507aae1fcdc02308b3a09034111b4c24071
Signed-off-by: Scott Peterson <scott.d.peterson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>