aoe: reserve enough headroom on skbs

Some network drivers use a non default hard_header_len

Transmitted skb should take into account dev->hard_header_len, or risk
crashes or expensive reallocations.

In the case of aoe, lets reserve MAX_HEADER bytes.

David reported a crash in defxx driver, solved by this patch.
Reported-by: NDavid Oostdyk <daveo@ll.mit.edu>
Tested-by: NDavid Oostdyk <daveo@ll.mit.edu>
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Ed Cashin <ecashin@coraid.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/block/aoe/aoecmd.c

@@ -51,8 +51,9 @@ new_skb(ulong len)
 {
 	struct sk_buff *skb;
 
-	skb = alloc_skb(len, GFP_ATOMIC);
+	skb = alloc_skb(len + MAX_HEADER, GFP_ATOMIC);
 	if (skb) {
+		skb_reserve(skb, MAX_HEADER);
 		skb_reset_mac_header(skb);
 		skb_reset_network_header(skb);
 		skb->protocol = __constant_htons(ETH_P_AOE);