libnvdimm/pmem: Fix blk_cleanup_disk() usage

libnvdimm/pmem: Fix blk_cleanup_disk() usage

The queue_to_disk() helper can not be used after del_gendisk()
communicate @disk via the pgmap->owner.

Otherwise, queue_to_disk() returns NULL resulting in the splat below.

 Kernel attempted to read user page (330) - exploit attempt? (uid: 0)
 BUG: Kernel NULL pointer dereference on read at 0x00000330
 Faulting instruction address: 0xc000000000906344
 Oops: Kernel access of bad area, sig: 11 [#1]
 [..]
 NIP [c000000000906344] pmem_pagemap_cleanup+0x24/0x40
 LR [c0000000004701d4] memunmap_pages+0x1b4/0x4b0
 Call Trace:
 [c000000022cbb9c0] [c0000000009063c8] pmem_pagemap_kill+0x28/0x40 (unreliable)
 [c000000022cbb9e0] [c0000000004701d4] memunmap_pages+0x1b4/0x4b0
 [c000000022cbba90] [c0000000008b28a0] devm_action_release+0x30/0x50
 [c000000022cbbab0] [c0000000008b39c8] release_nodes+0x2f8/0x3e0
 [c000000022cbbb60] [c0000000008ac440] device_release_driver_internal+0x190/0x2b0
 [c000000022cbbba0] [c0000000008a8450] unbind_store+0x130/0x170

Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Fixes: 87eb73b2ca7c ("nvdimm-pmem: convert to blk_alloc_disk/blk_cleanup_disk")
Link: http://lore.kernel.org/r/DFB75BA8-603F-4A35-880B-C5B23EF8FA7D@linux.vnet.ibm.com
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Link: https://lore.kernel.org/r/162310994435.1571616.334551212901820961.stgit@dwillia2-desk3.amr.corp.intel.com
[axboe: fold in compile warning fix]
Signed-off-by: Jens Axboe <axboe@kernel.dk>