block: allow end_io based requests in the completion batch handling

With end_io handlers now being able to potentially pass ownership of
the request upon completion, we can allow requests with end_io handlers
in the batch completion handling.

Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Co-developed-by: Stefan Roesch <shr@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>