Discard blocks when removing a thin device

dmsetup does not discard blocks when removing a thin device. The unused blocks
are reused by the thin-pool, but will remain allocated in the underlying
device indefinitely. For loop device backed thin-pools, this results in
"lost" disk space in the underlying file system as the blocks remain allocated
in the loop device's backing file.

This change adds an option, discard_blocks, to the devmapper snapshotter which
causes the snapshotter to issue blkdiscard ioctls on the thin device before
removal. With this option enabled, loop device setups will see disk space
return to the underlying filesystem immediately on exiting a container.

Fixes #5691

Signed-off-by: Kern Walster <walster@amazon.com>

snapshots/devmapper/pool_device_test.go

@@ -85,6 +85,7 @@ func TestPoolDevice(t *testing.T) {
 		RootPath:           tempDir,
 		BaseImageSize:      "16mb",
 		BaseImageSizeBytes: 16 * 1024 * 1024,
+		DiscardBlocks:      true,
 	}
 
 	pool, err := NewPoolDevice(ctx, config)