ubifs: Fixed print foramt mismatch in ubifs

fs/ubifs/super.c: function mount_ubifs:
the format specifier "lld" need arg type "long long",
but the according arg "old_idx_sz" has type
"unsigned long long"

Signed-off-by: Fangping Liang <liangfangping@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>