raid5-ppl: stop using bio_devname

Use the %pg format specifier to save on stack consuption and code size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Song Liu <song@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20220304180105.409765-9-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>