net: Fix warnings caused by MAX_SKB_FRAGS change.

After commit a715dea3 ("net: Always
allocate at least 16 skb frags regardless of page size"), the value
of MAX_SKB_FRAGS can now take on either an "unsigned long" or an
"int" value.

This causes warnings like:

net/packet/af_packet.c: In function ‘tpacket_fill_skb’:
net/packet/af_packet.c:948: warning: format ‘%lu’ expects type ‘long unsigned int’, but argument 2 has type ‘int’

Fix by forcing the constant to be unsigned long, otherwise we have
a situation where the type of a system wide constant is variable.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -126,7 +126,7 @@ struct sk_buff;
  * GRO uses frags we allocate at least 16 regardless of page size.
  */
 #if (65536/PAGE_SIZE + 2) < 16
-#define MAX_SKB_FRAGS 16
+#define MAX_SKB_FRAGS 16UL
 #else
 #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
 #endif