md: release allocated bitset sync_set

Patch fixes kmemleak on md_stop() path used likely only by dm-raid wrapper.
Code of md is using  mddev_put() where both bitsets are released however this
freeing is not shared.

Also set NULL to bio_set and sync_set pointers just like mddev_put is
doing.
Signed-off-by: NZdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: NShaohua Li <shli@fb.com>

drivers/md/md.c

@@ -5852,8 +5852,14 @@ void md_stop(struct mddev *mddev)
 	 * This is called from dm-raid
 	 */
 	__md_stop(mddev);
-	if (mddev->bio_set)
+	if (mddev->bio_set) {
 		bioset_free(mddev->bio_set);
+		mddev->bio_set = NULL;
+	}
+	if (mddev->sync_set) {
+		bioset_free(mddev->sync_set);
+		mddev->sync_set = NULL;
+	}
 }
 
 EXPORT_SYMBOL_GPL(md_stop);