We have a test case like block/001 in blktests, which will create a scsi
device by loading scsi_debug module and then try to delete the device by
sysfs interface. At the same time, it may remove the scsi_debug module.

And getting a invalid paging request BUG_ON as following:

[   34.625854] BUG: unable to handle page fault for address: ffffffffa0016bb8
[   34.629189] Oops: 0000 [#1] SMP PTI
[   34.629618] CPU: 1 PID: 450 Comm: bash Tainted: G        W         5.4.0-rc3+ #473
[   34.632524] RIP: 0010:scsi_proc_hostdir_rm+0x5/0xa0
[   34.643555] CR2: ffffffffa0016bb8 CR3: 000000012cd88000 CR4: 00000000000006e0
[   34.644545] Call Trace:
[   34.644907]  scsi_host_dev_release+0x6b/0x1f0
[   34.645511]  device_release+0x74/0x110
[   34.646046]  kobject_put+0x116/0x390
[   34.646559]  put_device+0x17/0x30
[   34.647041]  scsi_target_dev_release+0x2b/0x40
[   34.647652]  device_release+0x74/0x110
[   34.648186]  kobject_put+0x116/0x390
[   34.648691]  put_device+0x17/0x30
[   34.649157]  scsi_device_dev_release_usercontext+0x2e8/0x360
[   34.649953]  execute_in_process_context+0x29/0x80
[   34.650603]  scsi_device_dev_release+0x20/0x30
[   34.651221]  device_release+0x74/0x110
[   34.651732]  kobject_put+0x116/0x390
[   34.652230]  sysfs_unbreak_active_protection+0x3f/0x50
[   34.652935]  sdev_store_delete.cold.4+0x71/0x8f
[   34.653579]  dev_attr_store+0x1b/0x40
[   34.654103]  sysfs_kf_write+0x3d/0x60
[   34.654603]  kernfs_fop_write+0x174/0x250
[   34.655165]  __vfs_write+0x1f/0x60
[   34.655639]  vfs_write+0xc7/0x280
[   34.656117]  ksys_write+0x6d/0x140
[   34.656591]  __x64_sys_write+0x1e/0x30
[   34.657114]  do_syscall_64+0xb1/0x400
[   34.657627]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   34.658335] RIP: 0033:0x7f156f337130

During deleting scsi target, the scsi_debug module have been removed. Then,
sdebug_driver_template belonged to the module cannot be accessd, resulting
in scsi_proc_hostdir_rm() BUG_ON.

To fix the bug, we add scsi_device_get() in sdev_store_delete() to try to
increase refcount of module, avoiding the module been removed.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20191015130556.18061-1-yuyufen@huawei.comSigned-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>