ovl: simplify i_ino initialization

mainline inclusion
from mainline-v5.7-rc1
commit 62c832ed
category: bugfix
bugzilla: NA
CVE: NA

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Move i_ino initialization to ovl_inode_init() to avoid the dance of setting
i_ino in ovl_fill_inode() sometimes on the first call and sometimes on the
seconds call.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Zheng Liang <zhengliang6@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>