ubd: refactor the interrupt handler

[ Upstream commit 5db755fb ]

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>
Stable-dep-of: 31ade7d4 ("ubd: untagle discard vs write zeroes not support handling")
Signed-off-by: Sasha Levin <sashal@kernel.org>