Fix relabeling local volume source dir

In case a volume is specified via Mounts API, and SELinux is enabled,
the following error happens on container start:

> $ docker volume create testvol
> $ docker run --rm --mount source=testvol,target=/tmp busybox true
> docker: Error response from daemon: error setting label on mount
> source '': no such file or directory.

The functionality to relabel the source of a local mount specified via
Mounts API was introduced in commit 5bbf5cc and later broken by commit
e4b6adc, which removed setting mp.Source field.

With the current data structures, the host dir is already available in
v.Mountpoint, so let's just use it.

Fixes: e4b6adc
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/volumes.go

@@ -210,6 +210,8 @@ func (daemon *Daemon) registerMountPoints(container *container.Container, hostCo
 			mp.Name = v.Name
 			mp.Driver = v.Driver
 
+			// need to selinux-relabel local mounts
+			mp.Source = v.Mountpoint
 			if mp.Driver == volume.DefaultDriverName {
 				setBindModeIfNull(mp)
 			}