tun: Add missing bpf_net_ctx_clear() in do_xdp_generic()

There are cases where do_xdp_generic returns bpf_net_context without
clearing it. This causes various memory corruptions, so the missing
bpf_net_ctx_clear must be added.

Reported-by:  <syzbot+44623300f057a28baf1e@syzkaller.appspotmail.com>
Fixes: fecef4cd

 ("tun: Assign missing bpf_net_context.")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reported-by:  <syzbot+3c2b6d5d4bec3b904933@syzkaller.appspotmail.com>
Reported-by:  <syzbot+707d98c8649695eaf329@syzkaller.appspotmail.com>
Reported-by:  <syzbot+c226757eb784a9da3e8b@syzkaller.appspotmail.com>
Reported-by:  <syzbot+61a1cfc2b6632363d319@syzkaller.appspotmail.com>
Reported-by:  <syzbot+709e4c85c904bcd62735@syzkaller.appspotmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>