The PCIE Data Object Exchange (DOE) mailbox is a protocol run over
configuration cycles.  It assumes one initiator at a time.  While the
kernel has control of the mailbox user space writes could interfere with
the kernel access.

Mark DOE mailbox config space exclusive when iterated by the CXL driver.
Signed-off-by: NIra Weiny <ira.weiny@intel.com>
Reviewed-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220926215711.2893286-3-ira.weiny@intel.comSigned-off-by: NDan Williams <dan.j.williams@intel.com>