ovl: use a private non-persistent ino pool

mainline inclusion
from mainline-v5.7-rc1
commit 4d314f78
category: bugfix
bugzilla: NA
CVE: NA

-------------------------------------------------

There is no reason to deplete the system's global get_next_ino() pool for
overlay non-persistent inode numbers and there is no reason at all to
allocate non-persistent inode numbers for non-directories.

For non-directories, it is much better to leave i_ino the same as real
i_ino, to be consistent with st_ino/d_ino.

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>