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: Xiang Gao <qasdfgtyuiop@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>