openvswitch: Add eventmask support to CT action.

Add a new optional conntrack action attribute OVS_CT_ATTR_EVENTMASK,
which can be used in conjunction with the commit flag
(OVS_CT_ATTR_COMMIT) to set the mask of bits specifying which
conntrack events (IPCT_*) should be delivered via the Netfilter
netlink multicast groups.  Default behavior depends on the system
configuration, but typically a lot of events are delivered.  This can be
very chatty for the NFNLGRP_CONNTRACK_UPDATE group, even if only some
types of events are of interest.

Netfilter core init_conntrack() adds the event cache extension, so we
only need to set the ctmask value.  However, if the system is
configured without support for events, the setting will be skipped due
to extension not being found.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>