eulerfs: add error handling for nv_init()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I78RUK


CVE: NA

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Currently nv_init() doesn't handle errors, null-ptr-dereference will be
triggered if errors occur.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>