v2 ixgbe: Update packet buffer reservation to correct fdir headroom size

This change fixes an issue in which the incorrect amount of headroom was
being reserved for flow director filters.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>