netkit: Assign missing bpf_net_context

In the Linux kernel, the following vulnerability has been resolved:

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.