net: only use a single page of slop in MAX_SKB_FRAGS

In order to accommodate a 64K buffer we need 64K/PAGE_SIZE plus one more page
in order to allow for a buffer which does not start on a page boundary.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -128,13 +128,17 @@ struct sk_buff_head {
 
 struct sk_buff;
 
-/* To allow 64K frame to be packed as single skb without frag_list. Since
- * GRO uses frags we allocate at least 16 regardless of page size.
+/* To allow 64K frame to be packed as single skb without frag_list we
+ * require 64K/PAGE_SIZE pages plus 1 additional page to allow for
+ * buffers which do not start on a page boundary.
+ *
+ * Since GRO uses frags we allocate at least 16 regardless of page
+ * size.
  */
-#if (65536/PAGE_SIZE + 2) < 16
+#if (65536/PAGE_SIZE + 1) < 16
 #define MAX_SKB_FRAGS 16UL
 #else
-#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
+#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)
 #endif
 
 typedef struct skb_frag_struct skb_frag_t;