fs: allow clone_private_mount() for a path on real rootfs

Mounting overlayfs with a directory on real rootfs (initramfs)
as upperdir has failed with following message since commit
db04662e ("fs: allow detached mounts in clone_private_mount()").

  [    4.080134] overlayfs: failed to clone upperpath

Overlayfs mount uses clone_private_mount() to create internal mount
for the underlying layers.

The commit made clone_private_mount() reject real rootfs because
it does not have a parent mount and is in the initial mount namespace,
that is not an anonymous mount namespace.

This issue can be fixed by modifying the permission check
of clone_private_mount() following [1].

Reviewed-by: Christian Brauner <brauner@kernel.org>
Fixes: db04662e ("fs: allow detached mounts in clone_private_mount()")
Link: https://lore.kernel.org/all/20250514190252.GQ2023217@ZenIV/ [1]
Link: https://lore.kernel.org/all/20250506194849.GT2023217@ZenIV/


Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Kazuma Kondo <kazuma-kondo@nec.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>