block: allow end_io based requests in the completion batch handling

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 <[email protected]>
Reviewed-by: Keith Busch <[email protected]>
Co-developed-by: Stefan Roesch <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>