netkit: Assign missing bpf_net_context

During the introduction of struct bpf_net_context handling for
XDP-redirect, the netkit driver has been missed, which also requires it
because NETKIT_REDIRECT invokes skb_do_redirect() which is accessing the
per-CPU variables. Otherwise we see the following crash:

	BUG: kernel NULL pointer dereference, address: 0000000000000038
	bpf_redirect()
	netkit_xmit()
	dev_hard_start_xmit()

Set the bpf_net_context before invoking netkit_xmit() program within the
netkit driver.

Fixes: 401cb7da ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.")
Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20240912155620.1334587-1-leitao@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>