cxl/pmem: Fix nvdimm registration races

In the Linux kernel, the following vulnerability has been resolved:

cxl/pmem: Fix nvdimm registration races

A loop of the form:

while true; do modprobe cxl_pci; modprobe -r cxl_pci; done

...fails with the following crash signature:

BUG: kernel NULL pointer dereference, address: 0000000000000040

[..]

RIP: 0010:cxl_internal_send_cmd+0x5/0xb0 [cxl_core]

[..]

Call Trace:

<TASK>

cxl_pmem_ctl+0x121/0x240 [cxl_pmem]

nvdimm_get_config_data+0xd6/0x1a0 [libnvdimm]

nd_label_data_init+0x135/0x7e0 [libnvdimm]

nvdimm_probe+0xd6/0x1c0 [libnvdimm]

nvdimm_bus_probe+0x7a/0x1e0 [libnvdimm]

really_probe+0xde/0x380

__driver_probe_device+0x78/0x170

driver_probe_device+0x1f/0x90

__device_attach_driver+0x85/0x110

bus_for_each_drv+0x7d/0xc0

__device_attach+0xb4/0x1e0

bus_probe_device+0x9f/0xc0

device_add+0x445/0x9c0

nd_async_device_register+0xe/0x40 [libnvdimm]

async_run_entry_fn+0x30/0x130

...namely that the bottom half of async nvdimm device registration runs

after the CXL has already torn down the context that cxl_pmem_ctl()

needs. Unlike the ACPI NFIT case that benefits from launching multiple

nvdimm device registrations in parallel from those listed in the table,

CXL is already marked PROBE_PREFER_ASYNCHRONOUS. So provide for a

synchronous registration path to preclude this scenario.