MDEV-31347 fil_ibd_create() may hijack the file handle of an old file

fil_ibd_create(): Hold fil_system.mutex until fil_node_t::find_metadata()
has completed, so that node->handle cannot be closed by a concurrent
thread. This race condition was introduced
in commit 10dd290b (MDEV-17380).

Tested by: Matthias Leich

storage/innobase/fil/fil0fil.cc

@@ -2001,8 +2001,8 @@ fil_ibd_create(
 						     FIL_TYPE_TABLESPACE,
 						     crypt_data, mode, true)) {
 		fil_node_t* node = space->add(path, file, size, false, true);
-		mysql_mutex_unlock(&fil_system.mutex);
 		IF_WIN(node->find_metadata(), node->find_metadata(file, true));
+		mysql_mutex_unlock(&fil_system.mutex);
 		mtr.start();
 		mtr.set_named_space(space);
 		ut_a(fsp_header_init(space, size, &mtr) == DB_SUCCESS);