loop: use lo->lo_disk for kobject_uevent

hulk inclusion
category: bugfix
bugzilla: 189922, https://gitee.com/openeuler/kernel/issues/I9PXKA



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The current implimentation of partitions for loop has several issues.
'loop_device' is at the gendisk level, but it uses 'lo_device' to store
block_device. A gendisk can have multiple block_devices, and 'lo_device'
can be set to anyone of them through loop_configure().

This usage can lead to a null-ptr-deref issue. If 'lo_device' is set to
a block_device of partition, this partition is closed and 'bd_openers' is
dec to 0, the 'bd_disk' of that block_device will be set to NULL in
__blkdev_put(). In this case, Accessing 'lo_device->bd_disk' will trigger
the issue. The problem can be reproduced as follows:
  1. create loop device loop0 and create a partition loop0p1.
  2. submit the ioctl LOOP_CLR_FD by loop0.
  3. submit the ioctl LOOP_SET_FD by loop0p1.
  4. submit ioctl such as LOOP_SET_STATUS by loop0.

Fix it by using 'lo->lo_disk' instead of 'lo->lo_device->bd_disk' for
kobject_uevent().

Fixes: c3473c63 ("generate "change" uevent for loop device")
Signed-off-by: Li Nan <linan122@huawei.com>