korina: adjust headroom for new skb's also

This is copy and paste from the original driver. As skb_reserve() is
also called within korina_alloc_ring() when initially allocating the
receive descriptors, the same should be done when allocating new space
after passing an skb to upper layers.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>