xt_qtaguid: Fix panic caused by synack processing

xt_qtaguid: Fix panic caused by synack processing

In upstream commit ca6fb06518836ef9b65dc0aac02ff97704d52a05
(tcp: attach SYNACK messages to request sockets instead of
listener)
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ca6fb0651883

The building of synack messages was changed, which made it so
the skb->sk points to a casted request_sock. This is problematic,
as there is no sk_socket in a request_sock. So when the qtaguid_mt
function tries to access the sk->sk_socket, it accesses uninitialized
memory.

After looking at how other netfilter implementations handle this,
I realized there was a skb_to_full_sk() helper added, which the
xt_qtaguid code isn't yet using.

This patch adds its use, and resovles panics seen when accessing
uninitialzed memory when processing synack packets.

Reported-by: YongQin Liu <yongquin.liu@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>