packet lib: ipv6: Fix to calculate AH length

RFC2402 says:

    All IPv6 extension headers, as per RFC 1883, encode the "Hdr Ext Len" field by first
    subtracting 1 (64-bit word) from the header length (measured in 64-bit words).
    AH is an IPv6 extension header.  However, since its length is measured in 32-bit words,
    the "Payload Length" is calculated by subtracting 2 (32 bit words).

This patch fixes as follows:

    return (int(size) - 1) * 8 ->
    return (int(size) + 2) * 4
                      ^ ^    ^
And, this patch also fixes a default argument of length.

Signed-off-by: TAKAHASHI Minoru <takahashi.minoru7@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>