12771 11909 re-introduced lockstat panic

*** 492,501 ****
--- 492,502 ----
          lock
          cmpxchgq %rdx, (%rdi)                   /* try to drop read lock */
          jnz     rw_exit_wakeup
  .rw_read_exit_lockstat_patch_point:
          ret
+         movq    %gs:CPU_THREAD, %rcx            /* rcx = thread ptr */
          movq    %rdi, %rsi                      /* rsi = lock ptr */
          movl    $LS_RW_EXIT_RELEASE, %edi
          movl    $RW_READER, %edx
          jmp     lockstat_wrapper_arg
  .rw_not_single_reader: