[IPV6]: Make ipv6_skip_exthdrs take const skb arg.

include/net/ipv6.h

@@ -380,7 +380,7 @@ extern void			ipv6_push_frag_opts(struct sk_buff *skb,
 						    struct ipv6_txoptions *opt,
 						    u8 *proto);
 
-extern int			ipv6_skip_exthdr(struct sk_buff *, int start,
+extern int			ipv6_skip_exthdr(const struct sk_buff *, int start,
 					         u8 *nexthdrp, int len);
 
 extern int 			ipv6_ext_hdr(u8 nexthdr);

net/ipv6/exthdrs.c

@@ -694,7 +694,7 @@ int ipv6_ext_hdr(u8 nexthdr)
  * --ANK (980726)
  */
 
-int ipv6_skip_exthdr(struct sk_buff *skb, int start, u8 *nexthdrp, int len)
+int ipv6_skip_exthdr(const struct sk_buff *skb, int start, u8 *nexthdrp, int len)
 {
 	u8 nexthdr = *nexthdrp;