ubifs: dirty_cow_znode: Fix memleak in error handling path

ubifs: dirty_cow_znode: Fix memleak in error handling path

Following process will cause a memleak for copied up znode:

dirty_cow_znode
  zn = copy_znode(c, znode);
  err = insert_old_idx(c, zbr->lnum, zbr->offs);
  if (unlikely(err))
     return ERR_PTR(err);   // No one refers to zn.

Fix it by adding copied znode back to tnc, then it will be freed
by ubifs_destroy_tnc_subtree() while closing tnc.

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216705
Fixes: 1e51764a3c2a ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>