ipv6: Use ipv6_authlen for len in ipv6_skip_exthdr

In ipv6_skip_exthdr, the lengh of AH header is computed manually
as (hp->hdrlen+2)<<2. However, in include/linux/ipv6.h, a macro
named ipv6_authlen is already defined for exactly the same job. This
commit replaces the manual computation code with the macro.
Signed-off-by: NXiang Gao <qasdfgtyuiop@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/exthdrs_core.c

@@ -99,7 +99,7 @@ int ipv6_skip_exthdr(const struct sk_buff *skb, int start, u8 *nexthdrp,
 				break;
 			hdrlen = 8;
 		} else if (nexthdr == NEXTHDR_AUTH)
-			hdrlen = (hp->hdrlen+2)<<2;
+			hdrlen = ipv6_authlen(hp);
 		else
 			hdrlen = ipv6_optlen(hp);