Fix unmounts out of container export funcs

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

container.go

@@ -1416,9 +1416,12 @@ func (container *Container) ExportRw() (archive.Archive, error) {
 	if container.runtime == nil {
 		return nil, fmt.Errorf("Can't load storage driver for unregistered container %s", container.ID)
 	}
-	defer container.Unmount()
-
-	return container.runtime.Diff(container)
+	archive, err := container.runtime.Diff(container)
+	if err != nil {
+		container.Unmount()
+		return nil, err
+	}
+	return EofReader(archive, func() { container.Unmount() }), nil
 }
 
 func (container *Container) Export() (archive.Archive, error) {
@@ -1428,6 +1431,7 @@ func (container *Container) Export() (archive.Archive, error) {
 
 	archive, err := archive.Tar(container.basefs, archive.Uncompressed)
 	if err != nil {
+		container.Unmount()
 		return nil, err
 	}
 	return EofReader(archive, func() { container.Unmount() }), nil