Ensure proper value is used when computing reclaimable space

When Size was reverted to be equal to VirtualSize, the df command
formatter was not correctly updated to account for the change.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

command/formatter/disk_usage.go

@@ -196,7 +196,10 @@ func (c *diskUsageImagesContext) Reclaimable() string {
 	c.AddHeader(reclaimableHeader)
 	for _, i := range c.images {
 		if i.Containers != 0 {
-			used += i.Size
+			if i.VirtualSize == -1 || i.SharedSize == -1 {
+				continue
+			}
+			used += i.VirtualSize - i.SharedSize
 		}
 	}