macvlan: Rename fwd_priv to accel_priv and add accessor function

macvlan: Rename fwd_priv to accel_priv and add accessor function

This change renames the fwd_priv member to accel_priv as this more
accurately reflects the actual purpose of this value. In addition I am
adding an accessor which will allow us to further abstract this in the
future if needed.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>