ospfd: Make destination of p2p to multicast for LS-ACKS

ISSUE:

LSAcks (for directed acks) are being sent to neighbor's unicast address.

RFC 2328 says:

"The IP destination address for the packet is selected as
 follows.  On physical point-to-point networks, the IP
 destination is always set to the address AllSPFRouters"

Fix is to unconditionally set the destination address for LSAcks over
point-to-point links as AllSPFRouters. Quagga OSPF already has similar
change for OSPF DBD, LSUpdate and LSrequest packets.

Signed-off-by: Vipin Kumar <vipin@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by: Dinesh G Dutt <ddutt@cumulusnetworks.com>
diff --git a/ospfd/ospf_packet.c b/ospfd/ospf_packet.c
index b997f1b..f141822 100644
--- a/ospfd/ospf_packet.c
+++ b/ospfd/ospf_packet.c
@@ -3830,9 +3830,12 @@
   /* Set packet length. */
   op->length = length;
 
-  /* Set destination IP address. */
-  op->dst = dst;
-  
+  /* Decide destination address. */
+  if (oi->type == OSPF_IFTYPE_POINTOPOINT)
+    op->dst.s_addr = htonl (OSPF_ALLSPFROUTERS);
+  else
+    op->dst.s_addr = dst.s_addr;
+
   /* Add packet to the interface output queue. */
   ospf_packet_add (oi, op);