ipv6: Use ipv6_authlen for len

The length 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: yangxingwu <xingwu.yang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>