memstick/mspro_block: fix handling of read-only devices

Use set_disk_ro to propagate the read-only state to the block layer
instead of checking for it in ->open and leaking a reference in case
of a read-only device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220215094514.3828912-4-hch@lst.de


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