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.

Fixes #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,
 			},
 		}

test/apiv2/45-system.at

@@ -25,6 +25,26 @@ 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 '.Volumes[0].RefCount=0'
+t GET libpod/system/df '.Volumes[0].RefCount=0'
+
+# Make a container using the volume
+t POST containers/create Image=$IMAGE Volumes='{"/test":{}}' HostConfig='{"Binds":"foo1:/test"}' 201 \
+  .Id~[0-9a-f]\\{64\\}
+cid=$(jq -r '.Id' <<<"$output")
+
+# Verify that one container references the volume
+t GET system/df '.Volumes[0].RefCount=1'
+t GET libpod/system/df '.Volumes[0].RefCount=1'
+
+# Remove the container
+t DELETE containers/$cid?v=true 204
+
+# Verify that no containers reference the volume
+t GET system/df '.Volumes[0].RefCount=0'
+t GET libpod/system/df '.Volumes[0].RefCount=0'
+
 # Create two more volumes to test pruneing
 t POST libpod/volumes/create \
   Name=foo2 \