AF_PACKET: Check device down state before hard header callbacks.

If the device is down, invoking the device hard header callbacks
is not legal, so check it early.

Based upon a shaper OOPS report from Frederik Deweerdt.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>