openvswitch: Allow attaching helper in later commit

This patch allows to attach conntrack helper to a confirmed conntrack
entry.  Currently, we can only attach alg helper to a conntrack entry
when it is in the unconfirmed state.  This patch enables an use case
that we can firstly commit a conntrack entry after it passed some
initial conditions.  After that the processing pipeline will further
check a couple of packets to determine if the connection belongs to
a particular application, and attach alg helper to the connection
in a later stage.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>