Fix overlay use of rootdir and defer

Check for the rootDir first because the mergeDir may not exist if root
is present.

Also fix unmounting in the defer to make sure it does not have a
refcount.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

daemon/graphdriver/overlay/overlay.go

@@ -333,23 +333,22 @@ func (d *Driver) Get(id string, mountLabel string) (s string, err error) {
 	if _, err := os.Stat(dir); err != nil {
 		return "", err
 	}
+	// If id has a root, just return it
+	rootDir := path.Join(dir, "root")
+	if _, err := os.Stat(rootDir); err == nil {
+		return rootDir, nil
+	}
 	mergedDir := path.Join(dir, "merged")
 	if count := d.ctr.Increment(mergedDir); count > 1 {
 		return mergedDir, nil
 	}
 	defer func() {
 		if err != nil {
-			d.ctr.Decrement(mergedDir)
-			syscall.Unmount(mergedDir, 0)
+			if c := d.ctr.Decrement(mergedDir); c <= 0 {
+				syscall.Unmount(mergedDir, 0)
+			}
 		}
 	}()
-
-	// If id has a root, just return it
-	rootDir := path.Join(dir, "root")
-	if _, err := os.Stat(rootDir); err == nil {
-		return rootDir, nil
-	}
-
 	lowerID, err := ioutil.ReadFile(path.Join(dir, "lower-id"))
 	if err != nil {
 		return "", err