st: do not allocate a gendisk

st is a character driver and thus does not need to allocate a gendisk,
which is only used for file system-like block layer I/O on block
devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210816131910.615153-3-hch@lst.de


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