ppp: require CAP_NET_ADMIN in target netns for unattached ioctls

In the Linux kernel, the following vulnerability has been resolved:

ppp: require CAP_NET_ADMIN in target netns for unattached ioctls

/dev/ppp open is currently authorized against file->f_cred->user_ns, while unattached administrative ioctls operate on current->nsproxy->net_ns.

As a result, a local unprivileged user can create a new user namespace with CLONE_NEWUSER, gain CAP_NET_ADMIN only in that new user namespace, and still issue PPPIOCNEWUNIT, PPPIOCATTACH, or PPPIOCATTCHAN against an inherited network namespace.

Require CAP_NET_ADMIN in the user namespace that owns the target network namespace before handling unattached PPP administrative ioctls.

This preserves normal pppd operation in the network namespace it is actually privileged in, while rejecting the userns-only inherited-netns case.