Reworking of L2ARC persistency to simplify design.

*** 23,32 ****
--- 23,33 ----
   * Use is subject to license terms.
   */
  
  /*
   * Copyright (c) 2012 by Delphix. All rights reserved.
+  * Copyright 2013 Saso Kiselkov. All rights reserved.
   */
  
  /*      Copyright (c) 1984, 1986, 1987, 1988, 1989 AT&T */
  /*        All Rights Reserved   */
  
*** 130,139 ****
--- 131,148 ----
  #define ASSERT3U(x, y, z)       ((void)0)
  #define ASSERT3P(x, y, z)       ((void)0)
  #define ASSERT0(x)              ((void)0)
  #endif
  
+ /*
+  * Compile-time assertion. The condition 'x' must be constant.
+  */
+ #define CTASSERT(x)             _CTASSERT(x, __LINE__)
+ #define _CTASSERT(x, y)         __CTASSERT(x, y)
+ #define __CTASSERT(x, y) \
+         typedef char __compile_time_assertion__ ## y [(x) ? 1 : -1]
+ 
  #ifdef  _KERNEL
  
  extern void abort_sequence_enter(char *);
  extern void debug_enter(char *);