ethtool: ioctl: fix potential NULL deref in ethtool_set_coalesce()

ethtool_set_coalesce() now uses both the .get_coalesce() and .set_coalesce() callbacks. But the check for their availability is buggy, so changing the coalesce settings on a device where the driver provides only _one_ of the callbacks results in a NULL pointer dereference instead of an -EOPNOTSUPP.

Fix the condition so that the availability of both callbacks is ensured. This also matches the netlink code.

Note that reproducing this requires some effort - it only affects the legacy ioctl path, and needs a specific combination of driver options:
- have .get_coalesce() and .coalesce_supported but no .set_coalesce(), or
- have .set_coalesce() but no .get_coalesce(). Here eg. ethtool doesn't cause the crash as it first attempts to call ethtool_get_coalesce() and bails out on error.

---
 net/ethtool/ioctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 65e9bc1058b5..20bcf86970ff 100644
@@ -1719,7 +1719,7 @@ static noinline_for_stack int ethtool_set_coalesce(struct net_device *dev,
 	struct ethtool_coalesce coalesce;
 	int ret;
 
-	if (!dev->ethtool_ops->set_coalesce && !dev->ethtool_ops->get_coalesce)
+	if (!dev->ethtool_ops->set_coalesce || !dev->ethtool_ops->get_coalesce)
 		return -EOPNOTSUPP;
 
 	ret = dev->ethtool_ops->get_coalesce(dev, &coalesce, &kernel_coalesce,

2.32.0