re #13613 rb4516 Tunables needs volatile keyword

*** 21,30 ****
--- 21,33 ----
  /*
   * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   * Copyright 2011 Joyent, Inc.  All rights reserved.
   */
+ /*
+  * Copyright 2013 Nexenta Systems, Inc.  All rights reserved.
+  */
  
  /*
   * Copyright (c) 2016 by Delphix. All rights reserved.
   */
  
*** 159,170 ****
  
  /* Flags for bio_recycle() */
  #define BIO_HEADER      0x01
  #define BIO_MEM         0x02
  
! extern  int bufhwm;             /* User tunable - high water mark for mem  */
! extern  int bufhwm_pct;         /* ditto - given in % of physmem  */
  
  /*
   * The following routines allocate and free
   * buffers with various side effects.  In general the
   * arguments to an allocate routine are a device and
--- 162,173 ----
  
  /* Flags for bio_recycle() */
  #define BIO_HEADER      0x01
  #define BIO_MEM         0x02
  
! extern volatile int bufhwm;     /* User tunable - high water mark for mem  */
! extern volatile int bufhwm_pct; /* ditto - given in % of physmem  */
  
  /*
   * The following routines allocate and free
   * buffers with various side effects.  In general the
   * arguments to an allocate routine are a device and