Enhance the command "btrfs filesystem df" to show space usage information
for a mount point(s). It shows also an estimation of the space available,
on the basis of the current one used.
Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
[code moved under #if 0 instead of deletion]
Signed-off-by: David Sterba <dsterba@suse.cz>
