4469 DTrace helper tracing should be dynamic

*** 20,29 ****
--- 20,30 ----
   */
  
  /*
   * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2013 by Delphix. All rights reserved.
+  * Copyright (c) 2012, Joyent, Inc. All rights reserved.
   */
  
  /*
   * explicitly define DTRACE_ERRDEBUG to pull in definition of dtrace_errhash_t
   * explicitly define _STDARG_H to avoid stdarg.h/varargs.h u/k defn conflict
*** 1271,1293 ****
  
  int
  dtrace_helptrace_init(mdb_walk_state_t *wsp)
  {
          uint32_t next;
!         int enabled;
  
          if (wsp->walk_addr != NULL) {
                  mdb_warn("dtrace_helptrace only supports global walks\n");
                  return (WALK_ERR);
          }
  
!         if (mdb_readvar(&enabled, "dtrace_helptrace_enabled") == -1) {
!                 mdb_warn("couldn't read 'dtrace_helptrace_enabled'");
                  return (WALK_ERR);
          }
  
!         if (!enabled) {
                  mdb_warn("helper tracing is not enabled\n");
                  return (WALK_ERR);
          }
  
          if (mdb_readvar(&next, "dtrace_helptrace_next") == -1) {
--- 1272,1294 ----
  
  int
  dtrace_helptrace_init(mdb_walk_state_t *wsp)
  {
          uint32_t next;
!         uintptr_t buffer;
  
          if (wsp->walk_addr != NULL) {
                  mdb_warn("dtrace_helptrace only supports global walks\n");
                  return (WALK_ERR);
          }
  
!         if (mdb_readvar(&buffer, "dtrace_helptrace_buffer") == -1) {
!                 mdb_warn("couldn't read 'dtrace_helptrace_buffer'");
                  return (WALK_ERR);
          }
  
!         if (buffer == NULL) {
                  mdb_warn("helper tracing is not enabled\n");
                  return (WALK_ERR);
          }
  
          if (mdb_readvar(&next, "dtrace_helptrace_next") == -1) {