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 for UDPU,
because most of the switches are able to fragment and defragment packet
but it is problem for knet, because totempg is using maximum packet size
(65536 bytes) and when another header is added during retransmition,
then packet is too large.

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

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