btrfs: use a bool to track qgroup record insertion when adding ref head

We are using an integer as a boolean to track the qgroup record insertion
status when adding a delayed reference head. Since all we need is a
boolean, switch the type from int to bool to make it more obvious.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>