net: Increase default NET_SKB_PAD to 32.

Several devices need to insert some "pre headers" in front of the
main packet data when they transmit a packet.

Currently we allocate only 16 bytes of pad room and this ends up not
being enough for some types of hardware (NIU, usb-net, s390 qeth,
etc.)

So increase this to 32.

Note that drivers still need to check in their transmit routine
whether enough headroom exists, and if not use skb_realloc_headroom().
Tunneling, IPSEC, and other encapsulation methods can cause the
padding area to be used up.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -1287,7 +1287,7 @@ static inline int skb_network_offset(const struct sk_buff *skb)
  * The networking layer reserves some headroom in skb data (via
  * dev_alloc_skb). This is used to avoid having to reallocate skb data when
  * the header has to grow. In the default case, if the header has to grow
- * 16 bytes or less we avoid the reallocation.
+ * 32 bytes or less we avoid the reallocation.
  *
  * Unfortunately this headroom changes the DMA alignment of the resulting
  * network packet. As for NET_IP_ALIGN, this unaligned DMA is expensive
@@ -1295,11 +1295,11 @@ static inline int skb_network_offset(const struct sk_buff *skb)
  * perhaps setting it to a cacheline in size (since that will maintain
  * cacheline alignment of the DMA). It must be a power of 2.
  *
- * Various parts of the networking layer expect at least 16 bytes of
+ * Various parts of the networking layer expect at least 32 bytes of
  * headroom, you should not reduce this.
  */
 #ifndef NET_SKB_PAD
-#define NET_SKB_PAD	16
+#define NET_SKB_PAD	32
 #endif
 
 extern int ___pskb_trim(struct sk_buff *skb, unsigned int len);