block: avoid to touch unloaded module instance when opening bdev

disk->fops->owner is grabbed in blkdev_get_no_open() after the disk
kobject refcount is increased. This way can't make sure that
disk->fops->owner is still alive since del_gendisk() still can move
on if the kobject refcount of disk is grabbed by open() and
disk->fops->open() isn't called yet.

Fixes the issue by moving try_module_get() into blkdev_get_by_dev()
with ->open_mutex() held, then we can drain the in-progress open()
in del_gendisk(). Meantime new open() won't succeed because disk
becomes not alive.

This way is reasonable because blkdev_get_no_open() needn't to touch
disk->fops or defined callbacks.

Cc: Christoph Hellwig <hch@lst.de>
Cc: czhong@redhat.com
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211111020343.316126-1-ming.lei@redhat.com


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