OS-7088 cyclics corked on overlay socket with full queue

@@ -343,12 +343,20 @@
         int ret;
 
         /*
          * It'd be nice to be able to use MSG_MBLK_QUICKRELE, unfortunately,
          * that isn't actually supported by UDP at this time.
+         *
+         * Send with MSG_DONTWAIT to indicate clogged UDP sockets upstack.
          */
-        ret = ksocket_sendmblk(mux->omux_ksock, hdr, 0, &mp, kcred);
+        ret = ksocket_sendmblk(mux->omux_ksock, hdr, MSG_DONTWAIT, &mp, kcred);
+        /*
+         * NOTE: ksocket_sendmblk() may send partial packets downstack,
+         * returning what's not sent in &mp (i.e. mp pre-call might be a
+         * b_cont of mp post-call).  We can't hold up this message (it's a
+         * datagram), so we drop, and let the caller cope.
+         */
         if (ret != 0)
                 freemsg(mp);
 
         return (ret);
 }