Hopefully really final fix of Bug#11766591 59733: Possible deadlock

when buffered changes are to be discarded

sync_thread_add_level(level = SYNC_IBUF_INDEX_TREE): Relax a too strict
condition that the thread must not be holding locks below SYNC_FSP_PAGE.
It is perfectly valid to hold any latch above SYNC_IBUF_INDEX_TREE when
acquiring the insert buffer tree latch.

storage/innodb_plugin/sync/sync0sync.c

@@ -1258,8 +1258,7 @@ sync_thread_add_level(
 		break;
 	case SYNC_IBUF_INDEX_TREE:
 		if (sync_thread_levels_contain(array, SYNC_FSP)) {
-			ut_a(sync_thread_levels_g(
-				     array, SYNC_FSP_PAGE - 1, TRUE));
+			ut_a(sync_thread_levels_g(array, level - 1, TRUE));
 		} else {
 			ut_a(sync_thread_levels_g(
 				     array, SYNC_IBUF_TREE_NODE - 1, TRUE));