new util: 'btrfs'

new util: 'btrfs'

This commit introduces a new command called 'btrfs' for managing
a btrfs filesystem. 'btrfs' handles:
- snapshot/subvolume creation
- adding/removal of volume (ie: disk)
- defragment of a tree
- scan of a device searching a btrfs filesystem
- re-balancing of the chunk on the disks
- listing subvolumes and snapshots

This has also been updated to include the new defrag range ioctl.

Signed-off-by: Chris Mason <chris.mason@oracle.com>