overlay2: fix faulty errcheck

The change in 7a7357dae1bcccb17e9b2d4c7c8f5c025fce56ca inadvertently
changed the `defer` error code into a no-op. This restores its behavior
prior to that code change, and also introduces a little more error
logging.

Signed-off-by: Euan Kemp <euan.kemp@coreos.com>

daemon/graphdriver/overlay2/overlay.go

@@ -515,7 +515,7 @@ func (d *Driver) Remove(id string) error {
 }
 
 // Get creates and mounts the required file system for the given id and returns the mount path.
-func (d *Driver) Get(id, mountLabel string) (containerfs.ContainerFS, error) {
+func (d *Driver) Get(id, mountLabel string) (_ containerfs.ContainerFS, retErr error) {
 	d.locker.Lock(id)
 	defer d.locker.Unlock(id)
 	dir := d.dir(id)
@@ -538,9 +538,11 @@ func (d *Driver) Get(id, mountLabel string) (containerfs.ContainerFS, error) {
 		return containerfs.NewLocalContainerFS(mergedDir), nil
 	}
 	defer func() {
-		if err != nil {
+		if retErr != nil {
 			if c := d.ctr.Decrement(mergedDir); c <= 0 {
-				unix.Unmount(mergedDir, 0)
+				if mntErr := unix.Unmount(mergedDir, 0); mntErr != nil {
+					logrus.Errorf("error unmounting %v: %v", mergedDir, mntErr)
+				}
 			}
 		}
 	}()