btrfs-progs: Make csum tree rebuild works with extent tree rebuild.

btrfs-progs: Make csum tree rebuild works with extent tree rebuild.

Before this patch, csum tree rebuild will not work with extent tree
rebuild, since extent tree rebuild will only build up basic block
groups, but csum tree rebuild needs data extents to rebuild.
So if one use btrfsck with --init-csum-tree and --init-extent-tree, csum
tree will be empty and tons of "missing csum" error will be outputted.

This patch allows csum tree rebuild get its data from fs/subvol trees
using regular file extents (which is also the only one using csum tree
currently).

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[renamed to fill_csum_tree_from_one_fs_root]
Signed-off-by: David Sterba <dsterba@suse.cz>