ethtool: __ethtool_set_sg: check for function pointer before using it

__ethtool_set_sg does not check if dev->ethtool_ops->set_sg is defined
which can result in a NULL pointer dereference when ethtool is used to
change SG settings for drivers without SG support.

Signed-off-by: Roger Luethi <[e-mail]>
Reviewed-by: Ben Hutchings <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/ethtool.c

···
 {
 	int err;
 
+	if (!dev->ethtool_ops->set_sg)
+		return -EOPNOTSUPP;
+
 	if (data && !(dev->features & NETIF_F_ALL_CSUM))
 		return -EINVAL;