btrfs-progs: convert: Enhance record_file_blocks to handle reserved ranges

btrfs-progs: convert: Enhance record_file_blocks to handle reserved ranges

Enhance record_file_blocks() to handle reserved ranges.

Old file system can use the space in btrfs reserved ranges.
So we could not use the bytenr of old filesystem directly.

Thanks to previous patches, we have a full fs image in convert_root, and
it has already relocated the blocks in reserved ranges.
So here we just search the convert_root to get correct disk_bytenr and
use it.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>