net: qualcomm: rmnet: require CAP_NET_ADMIN in the real device netns for config ops

An rmnet device may be created with its real device in a different
netns than the rmnet device itself (rmnet_newlink() resolves it in
link_net), and the config paths below only check CAP_NET_ADMIN against
dev_net(dev), while mutating rmnet port state attached to the real
device:

- rmnet_changelink() rewrites the endpoint mux table and
  port->data_format and, via rmnet_vnd_update_dev_mtu(), can shrink the
  MTU of the rmnet endpoint netdevs.
- rmnet_add_bridge() and rmnet_del_bridge(), reachable via
  ndo_add_slave/ndo_del_slave through RTM_SETLINK IFLA_MASTER, flip
  port->rmnet_mode and port->bridge_ep on the real device's port; with
  bridge_ep pointing at a caller-owned device, rmnet_rx_handler() then
  forwards real-device ingress frames to it.
- rmnet_set_coalesce() rewrites the port aggregation parameters via
  ETHTOOL_SCOALESCE (ioctl) or ETHTOOL_MSG_COALESCE_SET (netlink),
  whose capability checks likewise only cover dev's netns.

A caller privileged only in the rmnet device's netns can therefore
rewrite the shared cellular data-path state owned by another netns, and
steer its ingress traffic.

Gate the rtnl paths with rtnl_dev_link_net_capable(), matching the
"require CAP_NET_ADMIN in the device netns for changelink" series
(vxlan/geneve, CVE-2026-68432), and gate the ethtool setter with
ns_capable() in the real device netns, mirroring the check dev_ethtool()
already applies to dev's netns.  Report the new rejections through
extack where one is available.

The gates cover the configuration paths only.  Teardown (RTM_DELLINK
reaching rmnet_dellink(), and rmnet_config_notify_cb() unregistering
the bridge when the slave device is deleted) is intentionally left
ungated: link deletion is normal netdev lifecycle behaviour, and the
changelink series this matches (vxlan/geneve, CVE-2026-68432) gated
only the configuration paths.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Abdifatah Suruur <suruurism@gmail.com>
Reviewed-by: Subash Abhinov Kasiviswanathan <subash.a.kasiviswanathan@oss.qualcomm.com>
Link: https://patch.msgid.link/20260915195642.1912-1-suruurism@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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