btrfs-progs: convert: Introduce functions to read used space

btrfs-progs: convert: Introduce functions to read used space

Before we do real convert, we need to read and build up used space cache
tree for later data/meta separate chunk layout.

This patch will iterate all used blocks in ext2 filesystem and record it
into cctx->used cache tree, for later use.

This provides the very basic of later btrfs-convert rework.

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