[NET]: Copy mac_len in skb_clone() as well

ANK says: "It is rarely used, that's wy it was not noticed.
But in the places, where it is used, it should be disaster."

Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>