This patch corrects a problem with the Reliable Data Gram cluster protocol
in which the cluster member can panic with "vl_unwire: page is not wired"
panic string.

crash> tf 
  >  0 stop_secondary_cpu   src/kernel/arch/alpha/cpu.c : 1205 
     1 panic                src/kernel/bsd/subr_prf.c : 1252 
     2 event_timeout        src/kernel/arch/alpha/cpu.c : 1971 
     3 printf               src/kernel/bsd/subr_prf.c : 940 
     4 panic                src/kernel/bsd/subr_prf.c : 1309 
     5 vl_unwire            src/kernel/vm/vm_vlock.c : 291 
     6 u_map_wire           src/kernel/vm/vm_umap.c : 1330 
     7 rdg_unwire_iodesc    src/kernel/rdg/rdg_kern.c : 1918 
     8 rdg_process_ctx_messages  src/kernel/rdg/rdg_kern.c : 5657 
     9 rdg_ctx_rcvq_scan_thread  src/kernel/rdg/rdg_kern.c : 6767 

