This syncs tree-checker.c from the kernel.  The main modification was to
add a open ctree flag to skip the deeper leaf checks, and plumbing this
through tree-checker.c.  We need this for things like fsck or
btrfs-image that need to work with slightly corrupted file systems, and
these checks simply make us unable to look at the corrupted blocks.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>