[NET]: migrate HARD_TX_LOCK to header file

HARD_TX_LOCK micro is a nice aggregation that could be used
in other spots. move it to netdevice.h
Also makes sure the previously superflous cpu arguement is used.
Thanks to DaveM for the suggestions.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>