ofproto-dpif: Expire fin_timeout actions when no previous timeout set.

ofproto-dpif: Expire fin_timeout actions when no previous timeout set.

Commit e503cc199 (ofproto: Optimise OpenFlow flow expiry) optimized
OpenFlow flow expiration by putting expirable flows on a list.  However,
the list is only configured at rule creation time.  If the rule is
created without a timeout, but is later set by the fin_timeout action,
it will never expire.  This commit adds the rule to the list when the
action is triggered.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>