kernel: deny swconfig set requests for unprivileged users

The swconfig kernel infrastructure fails to do any permissions checks when
changing settings. As such an ordinary user account on a device with a
switch can change switch settings without any special permissions.
Routers generally have few non-admin users so this isn't a big hole, but it
is a security hole. Likely the greatest danger is for multifunction devices
which have a lot of extra daemons, compromising a low-security daemon would
allow one to modify switch settings and cause the router/switch to appear to
lock-up (or cause other sorts of troublesome nyetwork behavior).

Implement a check for CAP_NET_ADMIN in swconfig_set_attr() and deny any
requests originating from user contexts lacking this capability.

Reported-by: Elliott Mitchell <ehem+openwrt@m5p.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

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