selinux: process labeled IPsec TCP SYN-ACK packets properly in selinux_ip_postroute()

selinux: process labeled IPsec TCP SYN-ACK packets properly in selinux_ip_postroute()

commit 5c6c26813a209e7075baf908e3ad81c1a9d389e8 upstream.

Due to difficulty in arriving at the proper security label for
TCP SYN-ACK packets in selinux_ip_postroute(), we need to check packets
while/before they are undergoing XFRM transforms instead of waiting
until afterwards so that we can determine the correct security label.

Reported-by: Janak Desai <Janak.Desai@gtri.gatech.edu>
Signed-off-by: Paul Moore <pmoore@redhat.com>
[bwh: Backported to 3.2:
 s/selinux_peerlbl_enabled()/netlbl_enabled() || selinux_xfrm_enabled()/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>