cxl/test: Remove ret_limit race condition in mock_get_event()

cxl/test: Remove ret_limit race condition in mock_get_event()

Commit 364ee9f3265e ("cxl/test: Enhance event testing") changed the
loop iterator in mock_get_event() from a static constant,
CXL_TEST_EVENT_CNT, to a dynamic global variable, ret_limit. The
intent was to vary the number of events returned per call to simulate
events occurring while logs are being read.

However, ret_limit is modified without synchronization. When multiple
threads call mock_get_event() concurrently, one thread may read
ret_limit, another thread may increment it, and the first thread's
loop condition and size calculation see and use the updated value.

This is visible during cxl_test module load when all memdevs are
initializing simultaneously, which includes getting event records. It
is not tied to the cxl-events.sh unit test specifically, as that
operates on a single memdev.

While no actual harm results (the buffer is always large enough and
the record count fields correctly reflect what was written), this is
a correctness issue. The race creates an inconsistent state within
mock_get_event() and adding variability based on a race appears
unintended.

Make ret_limit a local variable populated from an atomic counter. Each
call gets a stable value that won't change during execution. That
preserves the intended behavior of varying the return counts across
calls while eliminating the race condition.

This implementation uses "+ 1" to produce the full range of 1 to
CXL_TEST_EVENT_RET_MAX (4) records. Previously only 1, 2, 3 were
produced.

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/20251116013819.1713780-1-alison.schofield@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>