Somebody wanna review and/or test?

Changes:
* Define RX_OFFSET constant (value==2) for uses related to SKB 
allocation and skb_reserve() calls

* RX skb's are always allocated maximally-sized, since we don't know the 
size of an RX packet in advance.  This means that we always alloc and 
map RX skbs based on "np->rx_buf_sz + RX_OFFSET".  natsemi got this 
really wrong in refill_rx(), where it mapped skb->len just after 
dev_alloc_skb(), which was very incorrect.

* call skb_reserve() in refill_rx(), our main skb allocation function, 
just after dev_alloc_skb() returns successfully