net: tun: Enable XDP metadata support

Enable the support for the bpf_xdp_adjust_meta helper function for XDP
buffers initialized by the tun driver. This allows to reserve a metadata
area that is useful to pass any information from one XDP program to
another one, for example when using tail-calls.

Whether this helper function can be used in an XDP program depends on
how the xdp_buff was initialized. Most net drivers initialize the
xdp_buff in a way, that allows bpf_xdp_adjust_meta to be used. In case
of the tun driver, this is currently not the case.

There are two code paths in the tun driver that lead to a
bpf_prog_run_xdp and where metadata support should be enabled:

1. tun_build_skb, which is called by tun_get_user and is used when
   writing packets from userspace into the device. In this case, the
   xdp_buff created in tun_build_skb has no support for
   bpf_xdp_adjust_meta and calls of that helper function result in
   ENOTSUPP.

   For this code path, it's sufficient to set the meta_valid argument of
   the xdp_prepare_buff call. The reserved headroom is large enough
   already.

2. tun_xdp_one, which is called by tun_sendmsg which again is called by
   other drivers (e.g. vhost_net). When the TUN_MSG_PTR mode is used,
   another driver may pass a batch of xdp_buffs to the tun driver. In
   this case, that other driver is the one initializing the xdp_buff.

   See commit 043d222f ("tuntap: accept an array of XDP buffs
   through sendmsg()") for details.

   For now, the vhost_net driver is the only one using TUN_MSG_PTR and
   it already initializes the xdp_buffs with metadata support and
   sufficient headroom. But the tun driver disables it again, so the
   xdp_set_data_meta_invalid call has to be removed.

Signed-off-by: Marcus Wichelmann <marcus.wichelmann@hetzner-cloud.de>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/20250305213438.3863922-2-marcus.wichelmann@hetzner-cloud.de