If a crafted btrfs has missing block group items, it could cause
unexpected behavior and breaks our expectation on 1:1
chunk<->block group mapping.
Although we added block group -> chunk mapping check, we still need
chunk -> block group mapping check.
This patch will do extra check to ensure each
Reported in https://bugzilla.kernel.org/show_bug.cgi?id=199847, the
image has empty extent tree.
Although that image also has empty root tree, thus can be catched by
tree checker on empty leaves, but if a crafted btrfs has missing block
group items, it could still cause problem.
This patch will d
