idr: document exit conditions on idr_for_each_entry better

And some manual common subexpression elimination which may help the
compiler produce smaller code.

Signed-off-by: George Spelvin <linux@horizon.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>