clone_private_mnt(): make sure that caller has CAP_SYS_ADMIN in the right userns

What we want is to verify there is that clone won't expose something
hidden by a mount we wouldn't be able to undo.  "Wouldn't be able to undo"
may be a result of MNT_LOCKED on a child, but it may also come from
lacking admin rights in the userns of the namespace mount belongs to.

clone_private_mnt() checks the former, but not the latter.

There's a number of rather confusing CAP_SYS_ADMIN checks in various
userns during the mount, especially with the new mount API; they serve
different purposes and in case of clone_private_mnt() they usually,
but not always end up covering the missing check mentioned above.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Reported-by: "Orlando, Noah" <Noah.Orlando@deshaw.com>
Fixes: 427215d8 ("ovl: prevent private clone if bind mount is not allowed")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>