This patch crossports volumes.[ch] from btrfs-progs, including:
- btrfs_map_block()
  The core mechanism to map btrfs logical address to physical address.
  This version includes multi-device support, along with RAID56 support.

- btrfs_scan_one_device()
  This is the function to register one btrfs device to the list.
  This is the main part of the multi-device btrfs assembling process.
  Although we're not going to support multiple devices until U-Boot
  allows us to scan one device without actually opening it.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
[trini: Use %zu in a debug print to avoid warning]
Signed-off-by: Tom Rini <trini@konsulko.com>