nf: qtaguid: workaround xt_socket_get_sk() returning bad SKs.

nf: qtaguid: workaround xt_socket_get_sk() returning bad SKs.

(This is a direct cherry pick from 2.6.39: Id2a9912b)

* xt_socket_get_sk() returns invalid sockets when the sk_state is TCP_TIME_WAIT.
Added detection of time-wait.
* Added more constrained usage: qtaguid insures that xt_socket_get*_sk() is
not invoked for unexpected hooks or protocols (but I have not seen those
active at the point where the returned sk is bad).

Signed-off-by: JP Abgrall <jpa@google.com>
Change-Id: Id2a9912bb451a3e59d012fc55bbbd40fbb90693f