mm: shrinkers: fix double kfree on shrinker name

syzbot is reporting double kfree() at free_prealloced_shrinker() [1], for
destroy_unused_super() calls free_prealloced_shrinker() even if
prealloc_shrinker() returned an error. Explicitly clear shrinker name
when prealloc_shrinker() called kfree().

[[e-mail]: zero shrinker->name in all cases where shrinker->name is freed]
Link: https://lkml.kernel.org/r/YtgteTnQTgyuKUSY@castle
Link: https://syzkaller.appspot.com/bug?extid=8b481578352d4637f510 [1]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: e33c267ab70de424 ("mm: shrinkers: provide shrinkers with names")
Reported-by: syzbot <[e-mail]>
Signed-off-by: Tetsuo Handa <[e-mail]>
Acked-by: Roman Gushchin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/shrinker_debug.c

···
 	lockdep_assert_held(&shrinker_rwsem);
 
 	kfree_const(shrinker->name);
+	shrinker->name = NULL;
 
 	if (!shrinker->debugfs_entry)
 		return;

mm/vmscan.c

···
 		return -ENOMEM;
 
 	err = __prealloc_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 
 	return err;
 }
···
 {
 #ifdef CONFIG_SHRINKER_DEBUG
 	kfree_const(shrinker->name);
+	shrinker->name = NULL;
 #endif
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
 		down_write(&shrinker_rwsem);
···
 		return -ENOMEM;
 
 	err = __register_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 	return err;
 }
 #else