Move remount as private to the graph drivers

If this is at the root directory for the daemon you could unmount
somones filesystem when you stop docker and this is actually only needed
for the palces that the graph drivers mount the container's root
    filesystems.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #6221 from crosbymichael/no-unmount

Move remount as private to the graph drivers