md/raid5: Refactor for loop in raid5_make_request() into while loop

The for loop with retry label can be more cleanly expressed as a while
loop by moving the logical_sector increment into the success path.

No functional changes intended.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>