acpi, nfit: fix acpi_nfit_flush_probe() crash

We queue an on-stack work item to 'nfit_wq' and wait for it to complete
as part of a 'flush_probe' request. However, if the user cancels the
wait we need to make sure the item is flushed from the queue otherwise
we are leaving an out-of-scope stack address on the work list.

 BUG: unable to handle kernel paging request at ffffbcb3c72f7cd0
 IP: [<ffffffffa9413a7b>] __list_add+0x1b/0xb0
 [..]
 RIP: 0010:[<ffffffffa9413a7b>]  [<ffffffffa9413a7b>] __list_add+0x1b/0xb0
 RSP: 0018:ffffbcb3c7ba7c00  EFLAGS: 00010046
 [..]
 Call Trace:
  [<ffffffffa90bb11a>] insert_work+0x3a/0xc0
  [<ffffffffa927fdda>] ? seq_open+0x5a/0xa0
  [<ffffffffa90bb30a>] __queue_work+0x16a/0x460
  [<ffffffffa90bbb08>] queue_work_on+0x38/0x40
  [<ffffffffc0cf2685>] acpi_nfit_flush_probe+0x95/0xc0 [nfit]
  [<ffffffffc0cf25d0>] ? nfit_visible+0x40/0x40 [nfit]
  [<ffffffffa9571495>] wait_probe_show+0x25/0x60
  [<ffffffffa9546b30>] dev_attr_show+0x20/0x50

Fixes: 7ae0fa43 ("nfit, libnvdimm: async region scrub workqueue")
Cc: <stable@vger.kernel.org>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>