snapshots/devmapper: deactivate thin device after committed

1. reason to deactivate committed snapshot

The thin device will not be used for IO after committed,
and further thin snapshotting is OK using an inactive thin
device as origin. The benefits to deactivate are:
 - device is not unneccesary visible avoiding any unexpected IO;
 - save useless kernel data structs for maintaining active dm.

 Quote from kernel doc (Documentation/device-mapper/provisioning.txt):

"
  ii) Using an internal snapshot.

  Once created, the user doesn't have to worry about any connection
  between the origin and the snapshot.  Indeed the snapshot is no
  different from any other thinly-provisioned device and can be
  snapshotted itself via the same method.  It's perfectly legal to
  have only one of them active, and there's no ordering requirement on
  activating or removing them both.  (This differs from conventional
  device-mapper snapshots.)
"

2. an thinpool metadata bug is naturally removed

An problem happens when failed to suspend/resume origin thin device
when creating snapshot:

"failed to create snapshot device from parent vg0-mythinpool-snap-3"
error="failed to save initial metadata for snapshot "vg0-mythinpool-snap-19":
object already exists"

This issue occurs because when failed to create snapshot, the
snapshotter.store can be rollbacked, but the thin pool metadata
boltdb failed to rollback in PoolDevice.CreateSnapshotDevice(),
therefore metadata becomes inconsistent: the snapshotID is not
taken in snapshotter.store, but saved in pool metadata boltdb.

The cause is, in PoolDevice.CreateSnapshotDevice(), the defer calls
are invoked on "first-in-last-out" order. When the error happens
on the "resume device" defer call, the metadata is saved and
snapshot is created, which has no chance to be rollbacked.

Signed-off-by: Eric Ren <renzhen@linux.alibaba.com>

snapshots/devmapper/dmsetup/dmsetup.go

@@ -201,6 +201,13 @@ func RemoveDevice(deviceName string, opts ...RemoveDeviceOpt) error {
 	args = append(args, GetFullDevicePath(deviceName))
 
 	_, err := dmsetup(args...)
+	if err == unix.ENXIO {
+		// Ignore "No such device or address" error because we dmsetup
+		// remove with "deferred" option, there is chance for the device
+		// having been removed.
+		return nil
+	}
+
 	return err
 }
 
@@ -372,9 +379,13 @@ func parseDmsetupError(output string) string {
 		return ""
 	}
 
-	const failedSubstr = "failed: "
-
 	line := lines[0]
+	// Handle output like "Device /dev/mapper/snapshotter-suite-pool-snap-1 not found"
+	if strings.HasSuffix(line, "not found") {
+		return unix.ENXIO.Error()
+	}
+
+	const failedSubstr = "failed: "
 	idx := strings.LastIndex(line, failedSubstr)
 	if idx == -1 {
 		return ""