mm/ksm: fix -Wsometimes-uninitialized from clang-21 in advisor_mode_show()

After a recent change in clang to expose uninitialized warnings from const
variables [1], there is a false positive warning from the if statement in
advisor_mode_show().

  mm/ksm.c:3687:11: error: variable 'output' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
   3687 |         else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
        |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/ksm.c:3690:33: note: uninitialized use occurs here
   3690 |         return sysfs_emit(buf, "%s\n", output);
        |                                        ^~~~~~

Rewrite the if statement to implicitly make KSM_ADVISOR_NONE the else
branch so that it is obvious to the compiler that ksm_advisor can only be
KSM_ADVISOR_NONE or KSM_ADVISOR_SCAN_TIME due to the assignments in
advisor_mode_store().

Link: https://lkml.kernel.org/r/20250715-ksm-fix-clang-21-uninit-warning-v1-1-f443feb4bfc4@kernel.org
Fixes: 66790e9 ("mm/ksm: add sysfs knobs for advisor")
Signed-off-by: Nathan Chancellor <[email protected]>
Closes: ClangBuiltLinux#2100
Link: llvm/llvm-project@2464313 [1]
Acked-by: David Hildenbrand <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Stefan Roesch <[email protected]>
Cc: xu xin <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: nathanchance

mm/ksm.c

@@ -3669,10 +3669,10 @@ static ssize_t advisor_mode_show(struct kobject *kobj,
 {
 	const char *output;
 
-	if (ksm_advisor == KSM_ADVISOR_NONE)
-		output = "[none] scan-time";
-	else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+	if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
 		output = "none [scan-time]";
+	else
+		output = "[none] scan-time";
 
 	return sysfs_emit(buf, "%s\n", output);
 }