etherdevice: introduce help function eth_zero_addr()

a lot of code has either the memset or an inefficient copy
from a static array that contains the all-zeros Ethernet address.
Introduce help function eth_zero_addr() to fill an address with
all zeros, making the code clearer and allowing us to get rid of
some constant arrays.

Signed-off-by: Duan Jiong <djduanjiong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>