ubifs: do_kill_orphans: Fix a memory leak bug

If there are more than one valid snod on the sleb->nodes list,
do_kill_orphans will malloc ino more than once without releasing
previous ino's memory. Finally, it will trigger memory leak.

Fixes: ee1438ce

 ("ubifs: Check link count of inodes when...")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>