devmapper: Disable mount option "discard" by default

Right now devicemapper mounts thin device using online discards by default
and passes mount option "discard". Generally people discourage usage of
online discards as they can be a drain on performance. Instead it is 
recommended to use fstrim once in a while to reclaim the space.

In case of  containers, we recommend to keep data volumes separate. So
there might not be lot of rm, unlink operations going on and there might
not be lot of space being freed by containers. So it might not matter
much if we don't reclaim that free space in pool.

User can still pass mount option explicitly using dm.mountopt=discard to
enable discards if they would like to.

So this is more like setting the containers by default for better performance
instead of better space efficiency in pool. And user can change the behavior
if they don't like default behavior.

Reported-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -1366,11 +1366,7 @@ func (devices *DeviceSet) MountDevice(hash, path, mountLabel string) error {
 	options = joinMountOptions(options, devices.mountOptions)
 	options = joinMountOptions(options, label.FormatMountLabel("", mountLabel))
 
-	err = syscall.Mount(info.DevName(), path, fstype, flags, joinMountOptions("discard", options))
-	if err != nil && err == syscall.EINVAL {
-		err = syscall.Mount(info.DevName(), path, fstype, flags, options)
-	}
-	if err != nil {
+	if err := syscall.Mount(info.DevName(), path, fstype, flags, options); err != nil {
 		return fmt.Errorf("Error mounting '%s' on '%s': %s", info.DevName(), path, err)
 	}