btrfs-progs: check: introduce function to query tree block level

btrfs-progs: check: introduce function to query tree block level

Introduce function query_tree_block_level() to resolve tree block level
by the following method:
1) tree block backref level
2) tree block header level

And only when header level == backref level, and transid matches, it will
return the tree block level.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>