libnvdimm, pmem: fix a NULL pointer BUG in nd_pmem_notify

The following BUG was observed when nd_pmem_notify() was called
for a BTT device.  The use of a pmem_device pointer is not valid
with BTT.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
 IP: nd_pmem_notify+0x30/0xf0 [nd_pmem]
 Call Trace:
  nd_device_notify+0x40/0x50
  child_notify+0x10/0x20
  device_for_each_child+0x50/0x90
  nd_region_notify+0x20/0x30
  nd_device_notify+0x40/0x50
  nvdimm_region_notify+0x27/0x30
  acpi_nfit_scrub+0x341/0x590 [nfit]
  process_one_work+0x197/0x450
  worker_thread+0x4e/0x4a0
  kthread+0x109/0x140

Fix nd_pmem_notify() by setting nd_region and badblocks pointers
properly for BTT.

Cc: <stable@vger.kernel.org>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Fixes: 71999466

 ("libnvdimm: async notification support")
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>