Btrfs: add new defrag-range ioctl.

The btrfs defrag ioctl was limited to doing the entire file.  This
commit adds a new interface that can defrag a specific range inside
the file.

It can also force compression on the file, allowing you to selectively
compress individual files after they were created, even when mount -o
compress isn't turned on.

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