Revert "ubifs: dirty_cow_znode: Fix memleak in error handling path"

This reverts commit 122deabf (ubifs: dirty_cow_znode: Fix memleak
in error handling path).
After commit 122deabf applied, if insert_old_idx() failed, old
index neither exists in TNC nor in old-index tree. Which means that
old index node could be overwritten in layout_leb_in_gaps(), then
ubifs image will be corrupted in power-cut.

Fixes: 122deabf

 (ubifs: dirty_cow_znode: Fix memleak ... path)
Cc: stable@vger.kernel.org
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>