cxl/mbox: Restrict poison cmds to debugfs cxl_raw_allow_all

cxl/mbox: Restrict poison cmds to debugfs cxl_raw_allow_all

The Get, Inject, and Clear poison commands are not available for
direct user access because they require kernel driver controls to
perform safely.

Further restrict access to these commands by requiring the selection
of the debugfs attribute 'cxl_raw_allow_all' to enable in raw mode.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Link: https://lore.kernel.org/r/0e5cb41ffae2bab800957d3b9003eedfd0a2dfd5.1681838291.git.alison.schofield@intel.com
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Tested-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>