We should unregister the net_device first, before we give back
our reference on xdp_prog.  Otherwise xdp_prog may be freed
before .ndo_stop() disabled the datapath.  Found by code inspection.

Fixes: ecd63a02 ("nfp: add XDP support in the driver")
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NSimon Horman <simon.horman@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>