userns: Don't let unprivileged users trick privileged users into setting the id_map

When we require privilege for setting /proc/<pid>/uid_map or
/proc/<pid>/gid_map no longer allow an unprivileged user to
open the file and pass it to a privileged program to write
to the file.

Instead when privilege is required require both the opener and the
writer to have the necessary capabilities.

I have tested this code and verified that setting /proc/<pid>/uid_map
fails when an unprivileged user opens the file and a privielged user
attempts to set the mapping, that unprivileged users can still map
their own id, and that a privileged users can still setup an arbitrary
mapping.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
void free_user_ns(struct user_namespace *ns)
{
	struct user_namespace *parent;

	do {
		parent = ns->parent;
		proc_free_inum(ns->proc_inum);
		kmem_cache_free(user_ns_cachep, ns);
		ns = parent;
	} while (atomic_dec_and_test(&parent->count));
}

void free_user_ns(struct user_namespace *ns)
{
	struct user_namespace *parent;

	do {
		parent = ns->parent;
		proc_free_inum(ns->proc_inum);
		kmem_cache_free(user_ns_cachep, ns);
		ns = parent;
	} while (atomic_dec_and_test(&parent->count));
}
