btrfs-progs: filesystem: add 'du' command

btrfs-progs: filesystem: add 'du' command

'btrfs du' differs from regular du in that it will work to resolve which
blocks are shared between files in its list. This gives the user a more
accurate bytecount from which they can make decisions regarding management
of their file space.

We still print a total number of bytes counted (like regular du), but also
print the number of bytes which were found to have been shared amongst the
file set provided. From there it becomes trivial to calculate how much space
is exclusively owned.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
[ rename files to reflect the filesystem command group, add GPL v2
  file headers ]
Signed-off-by: David Sterba <dsterba@suse.com>