drivers/ide-cd: Handle missing driver data during status check gracefully

The 0day bot reports the below failure which happens occasionally, with
their randconfig testing (once every ~100 boots).  The Code points at
the private pointer ->driver_data being NULL, which hints at a race of
sorts where the private driver_data descriptor has disappeared by the
time we get to run the workqueue.

So let's check that pointer before we continue with issuing the command
to the drive.

This fix is of the brown paper bag nature but considering that IDE is
long deprecated, let's do that so that random testing which happens to
enable CONFIG_IDE during randconfig builds, doesn't fail because of
this.

Besides, failing the TEST_UNIT_READY command because the drive private
data is gone is something which we could simply do anyway, to denote
that there was a problem communicating with the device.

  BUG: unable to handle kernel NULL pointer dereference at 000001c0
  IP: cdrom_check_status
  *pde = 00000000
  Oops: 0000 [#1] SMP
  CPU: 1 PID: 155 Comm: kworker/1:2 Not tainted 4.14.0-rc8 #127
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
  Workqueue: events_freezable_power_ disk_events_workfn
  task: 4fe90980 task.stack: 507ac000
  EIP: cdrom_check_status+0x2c/0x90
  EFLAGS: 00210246 CPU: 1
  EAX: 00000000 EBX: 4fefec00 ECX: 00000000 EDX: 00000000
  ESI: 00000003 EDI: ffffffff EBP: 467a9340 ESP: 507aded0
   DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
  CR0: 80050033 CR2: 000001c0 CR3: 06e0f000 CR4: 00000690
  Call Trace:
   ? ide_cdrom_check_events_real
   ? cdrom_check_events
   ? disk_check_events
   ? process_one_work
   ? process_one_work
   ? worker_thread
   ? kthread
   ? process_one_work
   ? __kthread_create_on_node
   ? ret_from_fork
  Code: 53 83 ec 14 89 c3 89 d1 be 03 00 00 00 65 a1 14 00 00 00 89 44 24 10 31 c0 8b 43 18 c7 44 24 04 00 00 00 00 c7 04 24 00 00 00 00 <8a> 80 c0 01 00 00 c7 44 24 08 00 00 00 00 83 e0 03 c7 44 24 0c
  EIP: cdrom_check_status+0x2c/0x90 SS:ESP: 0068:507aded0
  CR2: 00000000000001c0
  ---[ end trace 2410e586dd8f88b2 ]---

Reported-and-tested-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>