Fixes Issue # 22992: docker commit failing.

1) docker create / run / start: this would create a snapshot device and mounts it onto the filesystem.
So the first time GET operation is called. it will create the rootfs directory and return the path to rootfs
2) Now when I do docker commit. It will call the GET operation second time. This time the refcount will check
that the count > 1 (count=2). so the rootfs already exists, it will just return the path to rootfs.

Earlier it was just returning the mp: /var/lib/docker/devicemapper/mnt/{ID} and hence the inconsistent paths error.

Signed-off-by: Shishir Mahajan <shishir.mahajan@redhat.com>

daemon/graphdriver/devmapper/driver.go

@@ -160,8 +160,9 @@ func (d *Driver) Remove(id string) error {
 // Get mounts a device with given id into the root filesystem
 func (d *Driver) Get(id, mountLabel string) (string, error) {
 	mp := path.Join(d.home, "mnt", id)
+	rootFs := path.Join(mp, "rootfs")
 	if count := d.ctr.Increment(mp); count > 1 {
-		return mp, nil
+		return rootFs, nil
 	}
 
 	uid, gid, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
@@ -186,7 +187,6 @@ func (d *Driver) Get(id, mountLabel string) (string, error) {
 		return "", err
 	}
 
-	rootFs := path.Join(mp, "rootfs")
 	if err := idtools.MkdirAllAs(rootFs, 0755, uid, gid); err != nil && !os.IsExist(err) {
 		d.ctr.Decrement(mp)
 		d.DeviceSet.UnmountDevice(id, mp)