ubd: refactor the interrupt handler

ubd: refactor the interrupt handler

Instead of a separate handler function that leaves no work in the
interrupt hanler itself, split out a per-request end I/O helper and
clean up the coding style and variable naming while we're at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://lore.kernel.org/r/20240531074837.1648501-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>