tools/testing/cxl: Add a sysfs attr to test poison inject limits

tools/testing/cxl: Add a sysfs attr to test poison inject limits

CXL devices may report a maximum number of addresses that a device
allows to be poisoned using poison injection. When cxl_test creates
mock CXL memory devices, it defaults to MOCK_INJECT_DEV_MAX==88 for
all mocked memdevs.

Add a sysfs attribute, poison_inject_max to module cxl_mock_mem so
that users can set a custom device injection limit. Fail, and return
-EBUSY, if the mock poison list is not empty.

/sys/bus/platform/drivers/cxl_mock_mem/poison_inject_max

A simple usage model is to set the attribute before running a test in
order to emulate a device's poison handling.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/0f25b2862b90013545450222d2199e435c6cc11a.1681874357.git.alison.schofield@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>