MDEV-6351 - --plugin=force has no effect for built-in plugins

mysqld didn't fail to start if a compiled-in plugin failed to initialize
(--xxx=FORCE behaving as --xxx=ON)

mysql-test/r/plugin_loaderr.result

@@ -8,3 +8,6 @@ PLUGIN_TYPE	STORAGE ENGINE
 PLUGIN_LIBRARY	NULL
 PLUGIN_LIBRARY_VERSION	NULL
 LOAD_OPTION	ON
+#
+# MDEV-6351 - --plugin=force has no effect for built-in plugins
+#

mysql-test/t/plugin_loaderr.test

@@ -8,3 +8,17 @@ SELECT
 PLUGIN_NAME,PLUGIN_STATUS,PLUGIN_TYPE,PLUGIN_LIBRARY,PLUGIN_LIBRARY_VERSION,LOAD_OPTION
 FROM INFORMATION_SCHEMA.PLUGINS WHERE plugin_name = 'innodb';
 
+--echo #
+--echo # MDEV-6351 - --plugin=force has no effect for built-in plugins
+--echo #
+--exec echo "wait" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--shutdown_server
+--source include/wait_until_disconnected.inc
+
+--error 1
+--exec $MYSQLD_CMD --innodb=force --innodb-page-size=6000
+
+--exec echo "restart" > $MYSQLTEST_VARDIR/tmp/mysqld.1.expect
+--enable_reconnect
+--source include/wait_until_connected_again.inc
+--disable_reconnect

sql/sql_plugin.cc

@@ -1612,7 +1612,7 @@ int plugin_init(int *argc, char **argv, int flags)
       if (plugin_initialize(&tmp_root, plugin_ptr, argc, argv, !is_myisam &&
                             (flags & PLUGIN_INIT_SKIP_INITIALIZATION)))
       {
-        if (mandatory)
+        if (plugin_ptr->load_option == PLUGIN_FORCE)
           goto err_unlock;
         plugin_ptr->state= PLUGIN_IS_DISABLED;
       }