tg3: remove open-coded skb_cow_head.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Nithin Nayak Sujir <nsujir@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>