memtest: use {READ,WRITE}_ONCE in memory scanning

memtest failed to find bad memory when compiled with clang.  So use
{WRITE,READ}_ONCE to access memory to avoid compiler over optimization.

Link: https://lkml.kernel.org/r/20240312080422.691222-1-qiang4.zhang@intel.com
Signed-off-by: Qiang Zhang <qiang4.zhang@intel.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: zhangqiang4

mm/memtest.c

@@ -51,10 +51,10 @@ static void __init memtest(u64 pattern, phys_addr_t start_phys, phys_addr_t size
 	last_bad = 0;
 
 	for (p = start; p < end; p++)
-		*p = pattern;
+		WRITE_ONCE(*p, pattern);
 
 	for (p = start; p < end; p++, start_phys_aligned += incr) {
-		if (*p == pattern)
+		if (READ_ONCE(*p) == pattern)
 			continue;
 		if (start_phys_aligned == last_bad + incr) {
 			last_bad += incr;