OS-7088 cyclics corked on overlay socket with full queue

*** 343,354 ****
          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.
           */
!         ret = ksocket_sendmblk(mux->omux_ksock, hdr, 0, &mp, kcred);
          if (ret != 0)
                  freemsg(mp);
  
          return (ret);
  }
--- 343,362 ----
          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, 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);
  }