btrfs: Add overview of device replace

#include "dev-replace.h"

#include "sysfs.h"

/*

 * Device replace overview

 *

 * [Objective]

 * To copy all extents (both new and on-disk) from source device to target

 * device, while still keeping the filesystem read-write.

 *

 * [Method]

 * There are two main methods involved:

 *

 * - Write duplication

 *

 *   All new writes will be written to both target and source devices, so even

 *   if replace gets canceled, sources device still contans up-to-date data.

 *

 *   Location:		handle_ops_on_dev_replace() from __btrfs_map_block()

 *   Start:		btrfs_dev_replace_start()

 *   End:		btrfs_dev_replace_finishing()

 *   Content:		Latest data/metadata

 *

 * - Copy existing extents

 *

 *   This happens by re-using scrub facility, as scrub also iterates through

 *   existing extents from commit root.

 *

 *   Location:		scrub_write_block_to_dev_replace() from

 *   			scrub_block_complete()

 *   Content:		Data/meta from commit root.

 *

 * Due to the content difference, we need to avoid nocow write when dev-replace

 * is happening.  This is done by marking the block group read-only and waiting

 * for NOCOW writes.

 *

 * After replace is done, the finishing part is done by swapping the target and

 * source devices.

 *

 *   Location:		btrfs_dev_replace_update_device_in_mapping_tree() from

 *   			btrfs_dev_replace_finishing()

 */

static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,

				       int scrub_ret);

static void btrfs_dev_replace_update_device_in_mapping_tree(