cxl/test: Assign overflow_err_count from log->nr_overflow

cxl/test: Assign overflow_err_count from log->nr_overflow

mock_get_event() uses an uninitialized local variable, nr_overflow, to
populate the overflow_err_count field. That results in incorrect
overflow_err_count values in mocked cxl_overflow trace events, such as
this case where the records are reported as 0 and should be non-zero:

[] cxl_overflow: memdev=mem7 host=cxl_mem.6 serial=7: log=Failure : 0 records from 1763228189130895685 to 1763228193130896180

Fix by using log->nr_overflow and remove the unused local variable.

A follow-up change was considered in cxl_mem_get_records_log() to
confirm that the overflow_err_count is non-zero when the overflow flag
is set [1]. Since the driver has no functional dependency on this
constraint, and a device that violates this specific requirement does
not cause incorrect driver behavior, no validation check is added.

[1] CXL 3.2, Table 8-65 Get Event Records Output Payload

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>> ---
Link: https://patch.msgid.link/20251116013036.1713313-1-alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>