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/README.md

@@ -26,6 +26,7 @@ The following configuration flags are supported:
   should be the same as in `/dev/mapper/` directory
 * `base_image_size` - defines how much space to allocate when creating the base device
 * `async_remove` - flag to async remove device using snapshot GC's cleanup callback
+* `discard_blocks` - whether to discard blocks when removing a device. This is especially useful for returning disk space to the filesystem when using loopback devices.
 
 Pool name and base image size are required snapshotter parameters.
 
@@ -93,6 +94,7 @@ cat << EOF
     pool_name = "${POOL_NAME}"
     root_path = "${DATA_DIR}"
     base_image_size = "10GB"
+    discard_blocks = true
 EOF
 ```