selinux: handle TCP SYN-ACK packets correctly in selinux_ip_output()

In selinux_ip_output() we always label packets based on the parent
socket.  While this approach works in almost all cases, it doesn't
work in the case of TCP SYN-ACK packets when the correct label is not
the label of the parent socket, but rather the label of the larval
socket represented by the request_sock struct.

Unfortunately, since the request_sock isn't queued on the parent
socket until *after* the SYN-ACK packet is sent, we can't lookup the
request_sock to determine the correct label for the packet; at this
point in time the best we can do is simply pass/NF_ACCEPT the packet.
It must be said that simply passing the packet without any explicit
labeling action, while far from ideal, is not terrible as the SYN-ACK
packet will inherit any IP option based labeling from the initial
connection request so the label *should* be correct and all our
access controls remain in place so we shouldn't have to worry about
information leaks.

Reported-by: Janak Desai <[email protected]>
Tested-by: Janak Desai <[email protected]>
Cc: [email protected]
Signed-off-by: Paul Moore <[email protected]>

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