ubifs: Fix memleak when insert_old_idx() failed

In the Linux kernel, the following vulnerability has been resolved:

ubifs: Fix memleak when insert_old_idx() failed

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.

Fetch a reproducer in [Link].

Function copy_znode() is split into 2 parts: resource allocation
and znode replacement, insert_old_idx() is split in similar way,
so resource cleanup could be done in error handling path without
corrupting metadata(mem & disk).
It's okay that old index inserting is put behind of add_idx_dirt(),
old index is used in layout_leb_in_gaps(), so the two processes do
not depend on each other.