block: document the holder argument to blkdev_get_by_path

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230620043536.707249-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>