libct/cg/fs/blkio: do not set weight == 0

For per-device weight, you can set weight and/or leaf weight.
The problem is, with the recent fix to use BFQ on cgroup v1,
if per-device weights are set, the code tries to set device
weight to blkio.bfq.weight, and the leaf weight to
blkio.leaf_weight_device. The latter file does not exist on
kernels v5.0, meaning one can not set any per-device weights
at all.

The fix is to only set weights if they are non-zero (i.e. set).

The test case will come in a following commit.

Fixes: 6339d8a
Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/cgroups/fs/blkio.go

@@ -41,11 +41,15 @@ func (s *BlkioGroup) Set(path string, r *configs.Resources) error {
 		}
 	}
 	for _, wd := range r.BlkioWeightDevice {
-		if err := cgroups.WriteFile(path, s.weightDeviceFilename, wd.WeightString()); err != nil {
-			return err
+		if wd.Weight != 0 {
+			if err := cgroups.WriteFile(path, s.weightDeviceFilename, wd.WeightString()); err != nil {
+				return err
+			}
 		}
-		if err := cgroups.WriteFile(path, "blkio.leaf_weight_device", wd.LeafWeightString()); err != nil {
-			return err
+		if wd.LeafWeight != 0 {
+			if err := cgroups.WriteFile(path, "blkio.leaf_weight_device", wd.LeafWeightString()); err != nil {
+				return err
+			}
 		}
 	}
 	for _, td := range r.BlkioThrottleReadBpsDevice {