libnvdimm/bus: Prevent duplicate device_unregister() calls

A multithreaded namespace creation/destruction stress test currently
fails with signatures like the following:

    sysfs group 'power' not found for kobject 'dax1.1'
    RIP: 0010:sysfs_remove_group+0x76/0x80
    Call Trace:
     device_del+0x73/0x370
     device_unregister+0x16/0x50
     nd_async_device_unregister+0x1e/0x30 [libnvdimm]
     async_run_entry_fn+0x39/0x160
     process_one_work+0x23c/0x5e0
     worker_thread+0x3c/0x390

    BUG: kernel NULL pointer dereference, address: 0000000000000020
    RIP: 0010:klist_put+0x1b/0x6c
    Call Trace:
     klist_del+0xe/0x10
     device_del+0x8a/0x2c9
     ? __switch_to_asm+0x34/0x70
     ? __switch_to_asm+0x40/0x70
     device_unregister+0x44/0x4f
     nd_async_device_unregister+0x22/0x2d [libnvdimm]
     async_run_entry_fn+0x47/0x15a
     process_one_work+0x1a2/0x2eb
     worker_thread+0x1b8/0x26e

Use the kill_device() helper to atomically resolve the race of multiple
threads issuing kill, device_unregister(), requests.

Reported-by: Jane Chu <jane.chu@oracle.com>
Reported-by: Erwin Tsaur <erwin.tsaur@oracle.com>
Fixes: 4d88a97a

 ("libnvdimm, nvdimm: dimm driver and base libnvdimm device-driver...")
Cc: <stable@vger.kernel.org>
Link: https://github.com/pmem/ndctl/issues/96
Tested-by: Tested-by: Jane Chu <jane.chu@oracle.com>
Link: https://lore.kernel.org/r/156341207846.292348.10435719262819764054.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>