UBIFS: fix return code

Fix to return -ENOMEM in the kmalloc() and d_make_root() error handling
case instead of 0, as done elsewhere in those functions.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>