net: Allow netdevices to specify needed head/tailroom

This patch adds needed_headroom/needed_tailroom members to struct
net_device and updates many places that allocate sbks to use them. Not
all of them can be converted though, and I'm sure I missed some (I
mostly grepped for LL_RESERVED_SPACE)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index b04d643..8fb134d 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -419,7 +419,7 @@
 		return;
 
 	size = arp_hdr_len(skb->dev);
-	send_skb = find_skb(np, size + LL_RESERVED_SPACE(np->dev),
+	send_skb = find_skb(np, size + LL_ALLOCATED_SPACE(np->dev),
 			    LL_RESERVED_SPACE(np->dev));
 
 	if (!send_skb)