Btrfs-progs: list snapshots by generation

Btrfs-progs: list snapshots by generation

The idea is that we usually use snapshot to backup/restore our data, and the
common way can be a cron script which makes lots of snapshots, so we can end
up with spending some time to find the latest snapshot to restore.

This adds a feature for 'btrfs subvolume list' to let it list snapshots by their
_created_ generation.

What we need to do is just to list them in descending order and get the latest
snapshot.  What's more, we can find the oldest snapshot as well by listing
snapshots in ascending order.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>