The IDE driver creates some passthru requests which never get
submitted to the block layer in such a way that blk_account_io_start()
gets called. However, the driver still calls __blk_mq_end_request() in
ide_end_rq() which will call blk_account_io_completion() which tries
to dereferences req->part which is never set. See ide_prep_sense() for
an example of where these requests come from.

To fix this, blk_account_io_completion() and blk_account_io_done()
should do nothing if req->part is not set.

The back trace of this bug is:

    BUG: kernel NULL pointer dereference, address: 000002ac
    #PF: supervisor write access in kernel mode
    #PF: error_code(0x0002) - not-present page
    *pde = 00000000
    Oops: 0002 [#1]
    CPU: 0 PID: 237 Comm: kworker/0:1H Not tainted
    5.4.0-rc2-00011-g48d9b0d4 #1
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1
    04/01/2014
    Workqueue: kblockd drive_rq_insert_work
    EIP: blk_account_io_completion+0x7a/0xf0
    Code: 89 54 24 08 31 d2 89 4c 24 04 31 c9 c7 04 24 02 00 00 00 c1 ee
    09 e8 f5 21 a6 ff e8 70 5c a7 ff 8b 53 60 8d 04 bd 00 00 00 00 <01> b4
    02 ac 02 00 00 8b 9a 88 02 00 00 85 db 74 11 85 d2 74 51 8b
    EAX: 00000000 EBX: f5b80000 ECX: 00000000 EDX: 00000000
    ESI: 00000000 EDI: 00000000 EBP: f3031e70 ESP: f3031e54
    DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068 EFLAGS: 00010046
    CR0: 80050033 CR2: 000002ac CR3: 03c25000 CR4: 000406d0
    Call Trace:
     <IRQ>
      blk_update_request+0x85/0x420
      ide_end_rq+0x38/0xa0
      ide_complete_rq+0x3d/0x70
      cdrom_newpc_intr+0x258/0xba0
      ide_intr+0x135/0x250
      __handle_irq_event_percpu+0x3e/0x250
      handle_irq_event_percpu+0x1f/0x50
      handle_irq_event+0x32/0x60
      handle_level_irq+0x6c/0x110
      handle_irq+0x72/0xa0
      </IRQ>
      do_IRQ+0x45/0xad
      common_interrupt+0x115/0x11c

Fixes: 48d9b0d4 ("block: account statistics for passthrough requests")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>