memstick/ms_block: simplify refcounting

Implement the ->free_disk method to free the msb_data structure only once
the last gendisk reference goes away instead of keeping a local refcount.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220215094514.3828912-3-hch@lst.de


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