tun: Assign missing bpf_net_context.

During the introduction of struct bpf_net_context handling for
XDP-redirect, the tun driver has been missed.
Jakub also pointed out that there is another call chain to
do_xdp_generic() originating from netif_receive_skb() and drivers may
use it outside from the NAPI context.

Set the bpf_net_context before invoking BPF XDP program within the TUN
driver. Set the bpf_net_context also in do_xdp_generic() if a xdp
program is available.

Reported-by:  <syzbot+0b5c75599f1d872bea6f@syzkaller.appspotmail.com>
Reported-by:  <syzbot+5ae46b237278e2369cac@syzkaller.appspotmail.com>
Reported-by:  <syzbot+c1e04a422bbc0f0f2921@syzkaller.appspotmail.com>
Fixes: 401cb7da

 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20240704144815.j8xQda5r@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>