[SECMARK]: Add secmark support to conntrack

Add a secmark field to IP and NF conntracks, so that security markings
on packets can be copied to their associated connections, and also
copied back to packets as required.  This is similar to the network
mark field currently used with conntrack, although it is intended for
enforcement of security policy rather than network policy.

Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>