print parent ID in btrfs suvolume list

There was some discussion on "where" subvolumes live in. Why do we not
simply print the parent ID for each subvolume in btrfs subvolume list?
This patch adds this functionality when called with parameter "-p".

Signed-off-by: Andreas Philipp <philipp.andreas@gmail.com>
Signed-off-by: Hugo Mills <hugo@carfax.org.uk>
4 files changed