cxl/mem: Fix shutdown order

cxl/mem: Fix shutdown order

Ira reports that removing cxl_mock_mem causes a crash with the following
trace:

 BUG: kernel NULL pointer dereference, address: 0000000000000044
 [..]
 RIP: 0010:cxl_region_decode_reset+0x7f/0x180 [cxl_core]
 [..]
 Call Trace:
  <TASK>
  cxl_region_detach+0xe8/0x210 [cxl_core]
  cxl_decoder_kill_region+0x27/0x40 [cxl_core]
  cxld_unregister+0x29/0x40 [cxl_core]
  devres_release_all+0xb8/0x110
  device_unbind_cleanup+0xe/0x70
  device_release_driver_internal+0x1d2/0x210
  bus_remove_device+0xd7/0x150
  device_del+0x155/0x3e0
  device_unregister+0x13/0x60
  devm_release_action+0x4d/0x90
  ? __pfx_unregister_port+0x10/0x10 [cxl_core]
  delete_endpoint+0x121/0x130 [cxl_core]
  devres_release_all+0xb8/0x110
  device_unbind_cleanup+0xe/0x70
  device_release_driver_internal+0x1d2/0x210
  bus_remove_device+0xd7/0x150
  device_del+0x155/0x3e0
  ? lock_release+0x142/0x290
  cdev_device_del+0x15/0x50
  cxl_memdev_unregister+0x54/0x70 [cxl_core]

This crash is due to the clearing out the cxl_memdev's driver context
(@cxlds) before the subsystem is done with it. This is ultimately due to
the region(s), that this memdev is a member, being torn down and expecting
to be able to de-reference @cxlds, like here:

static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)
...
                if (cxlds->rcd)
                        goto endpoint_reset;
...

Fix it by keeping the driver context valid until memdev-device
unregistration, and subsequently the entire stack of related
dependencies, unwinds.

Fixes: 9cc238c7a526 ("cxl/pci: Introduce cdevm_file_operations")
Reported-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Tested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>