datapath: Drop packets when interdev is not up

If the internal device is not up, it should drop received
packets. Sometimes it receive the broadcast or multicast
packets, and the ip protocol stack will casue more cpu
usage wasted.

Signed-off-by: Chunhe Li <lichunhe@huawei.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>