ipv4: tcp: unicast_sock should not land outside of TCP stack

ipv4: tcp: unicast_sock should not land outside of TCP stack

commit 79f76c5ee9e4506d (ipv4: tcp: remove per net tcp_sock) added a
selinux regression, reported and bisected by John Stultz

selinux_ip_postroute_compat() expect to find a valid sk->sk_security
pointer, but this field is NULL for unicast_sock

It turns out that unicast_sock are really temporary stuff to be able
to reuse  part of IP stack (ip_append_data()/ip_push_pending_frames())

Fact is that frames sent by ip_send_unicast_reply() should be orphaned
to not fool LSM.

Note IPv6 never had this problem, as tcp_v6_send_response() doesnt use a
fake socket at all. I'll probably implement tcp_v4_send_response() to
remove these unicast_sock in linux-3.7

Reported-by: John Stultz <johnstul@us.ibm.com>
Bisected-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Eric Paris <eparis@parisplace.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>