Ensure that the DF endpoint updated volume refcount

The field was already exposed already in the `system df` output
so this just required a bit of plumbing and testing.

As part of this, fix `podman systemd df` volume in-use logic.
Previously, volumes were only considered to be in use if the
container using them was running. This does not match Docker's
behavior, where a volume is considered in use as long as a
container exists that uses the volume, even if said container is
not running.

Fixes containers#15720

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

pkg/api/handlers/compat/system.go

@@ -76,7 +76,7 @@ func GetDiskUsage(w http.ResponseWriter, r *http.Request) {
 			Scope:      "local",
 			Status:     nil,
 			UsageData: &docker.VolumeUsageData{
-				RefCount: 1,
+				RefCount: int64(o.Links),
 				Size:     o.Size,
 			},
 		}

pkg/domain/infra/abi/system.go

@@ -333,7 +333,6 @@ func (ic *ContainerEngine) SystemDf(ctx context.Context, options entities.System
 	dfVolumes := make([]*entities.SystemDfVolumeReport, 0, len(vols))
 	for _, v := range vols {
 		var reclaimableSize uint64
-		var consInUse int
 		mountPoint, err := v.MountPoint()
 		if err != nil {
 			return nil, err
@@ -355,14 +354,9 @@ func (ic *ContainerEngine) SystemDf(ctx context.Context, options entities.System
 		if len(inUse) == 0 {
 			reclaimableSize = volSize
 		}
-		for _, viu := range inUse {
-			if cutil.StringInSlice(viu, runningContainers) {
-				consInUse++
-			}
-		}
 		report := entities.SystemDfVolumeReport{
 			VolumeName:      v.Name(),
-			Links:           consInUse,
+			Links:           len(inUse),
 			Size:            int64(volSize),
 			ReclaimableSize: int64(reclaimableSize),
 		}

test/apiv2/45-system.at

@@ -1,5 +1,4 @@
 # -*- sh -*-
-#
 # system related tests
 #
 
@@ -25,6 +24,24 @@ t GET system/df 200 '.Volumes[0].Name=foo1'
 
 t GET libpod/system/df 200 '.Volumes[0].VolumeName=foo1'
 
+# Verify that no containers reference the volume
+t GET system/df 200 '.Volumes[0].UsageData.RefCount=0'
+
+# Make a container using the volume
+podman pull $IMAGE &>/dev/null
+t POST containers/create Image=$IMAGE Volumes='{"/test":{}}' HostConfig='{"Binds":["foo1:/test"]}' 201 \
+  .Id~[0-9a-f]\\{64\\}
+cid=$(jq -r '.Id' <<<"$output")
+
+# Verify that one container references the volume
+t GET system/df 200 '.Volumes[0].UsageData.RefCount=1'
+
+# Remove the container
+t DELETE containers/$cid?v=true 204
+
+# Verify that no containers reference the volume
+t GET system/df 200 '.Volumes[0].UsageData.RefCount=0'
+
 # Create two more volumes to test pruneing
 t POST libpod/volumes/create \
   Name=foo2 \