cxl/pmem: Enforce keyctl ABI for PMEM security

Preclude the possibility of user tooling sending device secrets in the
clear into the kernel by marking the security commands as exclusive.
This mandates the usage of the keyctl ABI for managing the device
passphrase.

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/166993221008.1995348.11651567302609703175.stgit@dwillia2-xfh.jf.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>