Otherwise we end up with attempting to send packets from down devices
or to send oversized packets, which may cause unexpected driver/device
behaviour. Generic XDP has already done this check, so reuse the logic
in native XDP.

Fixes: 814abfab

 ("xdp: add bpf_redirect helper function")
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>