btrfs: reloc: add backref_cache::pending_edge and backref_cache::useless_node
These two new members will act the same as the existing local lists, @useless and @list in build_backref_tree(). Currently build_backref_tree() is only executed serially, thus moving such local list into backref_cache is still safe. Also since we're here, use list_first_entry() to replace a lot of list_entry() calls after !list_empty(). Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: Qu Wenruo <wqu@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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