tcp: MD5: Fix IPv6 signatures

Reported by Stefanos Harhalakis; although 2.6.27-rc1 talks to itself using IPv6
TCP MD5 packets just fine, Stefanos noted that tcpdump claimed that the
signatures were invalid.

I broke this in 49a72dfb

 ("tcp: Fix MD5
signatures for non-linear skbs"), it was just a typo.

Note that tcpdump will still sometimes claim that the signatures are incorrect.
A patch to tcpdump has been submitted for this[1].

[1] http://tinyurl.com/6a4fl2

Signed-off-by: Adam Langley <agl@imperialviolet.org>
Signed-off-by: David S. Miller <davem@davemloft.net>