Revert "lib/plist.c: enforce memory ordering in plist_check_list"

This reverts commit 7abcb84f95.

The introduction of WRITE_ONCE() calls for the 'prev' and 'next' variables
inside plist_check_list() was a misapplication.  WRITE_ONCE() is
fundamentally a compiler barrier designed to prevent compiler
optimizations (like caching or reordering) on shared memory locations. 
However, the variables 'prev' and 'next' are local, stack-allocated
pointers accessed only by the current thread's invocation of the function.

Since these pointers are thread-local and are never accessed concurrently,
applying WRITE_ONCE() to them is semantically incorrect and unnecessary. 
Furthermore, the use of WRITE_ONCE() on local variables prevents the
compiler from performing standard optimizations, such as keeping these
variables cached solely in CPU registers throughout the loop, potentially
introducing performance overhead.  Restore the conventional C assignment
for local loop variables, allowing the compiler to generate optimal code.

Link: https://lkml.kernel.org/r/20251113193413.499309-1-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: I Hsin Cheng <richard120310@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/plist.c

@@ -47,8 +47,8 @@ static void plist_check_list(struct list_head *top)
 
 	plist_check_prev_next(top, prev, next);
 	while (next != top) {
-		WRITE_ONCE(prev, next);
-		WRITE_ONCE(next, prev->next);
+		prev = next;
+		next = prev->next;
 		plist_check_prev_next(top, prev, next);
 	}
 }