totemsrp: Reduce MTU to left room second mcast

Messages sent during recovery phase are encapsulated so such message has
extra size of mcast structure. This is not so big problem,
because most of the switches are able to fragment and defragment packet,
but it looks risky to rely on such behavior.

Solution is to reduce mtu by 2 * sizeof (struct mcast).

(backported from master ee8b8993d98b3f6af9c058194228fc534fcd0796)

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>

exec/totemsrp.c

@@ -5127,7 +5127,7 @@ void main_iface_change_fn (
 }
 
 void totemsrp_net_mtu_adjust (struct totem_config *totem_config) {
-	totem_config->net_mtu -= sizeof (struct mcast);
+	totem_config->net_mtu -= 2 * sizeof (struct mcast);
 }
 
 void totemsrp_service_ready_register (