net/mlx4_en: Disbale GRO for incoming loopback/selftest packets

Packets which are sent from the selftest (ethtool) flow,
should not be passed to GRO stack but rather dropped by
the driver after validation. To achieve that, we disable
GRO for the duration of the selftest.

Fixes: dd65beac

 ("net/mlx4_en: Extend usage of napi_gro_frags")
Reported-by: Carol Soto <clsoto@linux.vnet.ibm.com>
Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>