This is the user mode part of the device replace patch series.
The command group "btrfs replace" is added with three commands:
- btrfs replace start srcdev|srcdevid targetdev [-Bfr] mount_point
- btrfs replace status mount_point [-1]
- btrfs replace cancel mount_point
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
