[IPV6]: Convert cork.hop_limit and cork.tclass into u8 instead of int.

Values of those fields are always between 0 and 255 (inclusive),
so use u8 and save some memory on 32bit systems.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

include/linux/ipv6.h

@@ -315,8 +315,8 @@ struct ipv6_pinfo {
 	struct sk_buff		*pktoptions;
 	struct {
 		struct ipv6_txoptions *opt;
-		int hop_limit;
-		int tclass;
+		u8 hop_limit;
+		u8 tclass;
 	} cork;
 };