macvlan: macvlan_count_rx shouldn't be static inline AND extern

It doesn't make sense to define macvlan_count_rx as a static inline and
then add a forward declaration after that as an extern. I am dropping the
extern declaration since it seems like it is something that likely got
missed when the function was made an inline.

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>