ubifs: Fix spelling mistakes

Fix some spelling mistakes in comments:
withoug  ==> without
numer  ==> number
aswell  ==> as well
referes  ==> refers
childs  ==> children
unnecesarry  ==> unnecessary

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>