Be far more judicious in the use of curzone-using macros.
(Merge and extra asserts by danmcd.)
Try to remove assumption that zone's root vnode is marked VROOT

*** 20,30 ****
   */
  
  /*
   * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright 2014 Igor Kozhukhov <ikozhukhov@gmail.com>.
!  * Copyright 2018 Joyent, Inc.
   * Copyright 2019 Nexenta Systems, Inc. All rights reserved.
   */
  
  #ifndef _SYS_ZONE_H
  #define _SYS_ZONE_H
--- 20,30 ----
   */
  
  /*
   * Copyright (c) 2003, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright 2014 Igor Kozhukhov <ikozhukhov@gmail.com>.
!  * Copyright 2019 Joyent, Inc.
   * Copyright 2019 Nexenta Systems, Inc. All rights reserved.
   */
  
  #ifndef _SYS_ZONE_H
  #define _SYS_ZONE_H
*** 780,793 ****
   */
  #define ZONE_SPECIALPID(x)       ((x) == 0 || (x) == 1)
  
  /*
   * A root vnode of the current zone.
   */
! #define ZONE_ROOTVP()   (curproc->p_zone->zone_rootvp)
  
  /*
   * Zone-safe version of thread_create() to be used when the caller wants to
   * create a kernel thread to run within the current zone's context.
   */
  extern kthread_t *zthread_create(caddr_t, size_t, void (*)(), void *, size_t,
      pri_t);
--- 780,808 ----
   */
  #define ZONE_SPECIALPID(x)       ((x) == 0 || (x) == 1)
  
  /*
   * A root vnode of the current zone.
+  *
+  * NOTE: It may be necessary (initialization time for file sharing where an
+  * NGZ loads a file-sharing kernel module that does zsd initialization) to NOT
+  * use this macro. One should ASSERT() that curzone == active ZSD (an
+  * ASSERTion that's not always true at ZSD initialization time) during regular
+  * use of this macro.
   */
! #define ZONE_ROOTVP()   (curzone->zone_rootvp)
  
  /*
+  * Since a zone's root isn't necessarily an actual filesystem boundary
+  * (i.e. VROOT may not be set on zone->zone_rootvp) we need to not assume it.
+  * This macro helps in checking if a vnode is the current zone's rootvp.
+  * NOTE:  Using the VN_ prefix, even though it's defined here in zone.h.
+  * NOTE2: See above warning about ZONE_ROOTVP().
+  */
+ #define VN_IS_CURZONEROOT(vp)   (VN_CMP(vp, ZONE_ROOTVP()))
+ 
+ /*
   * Zone-safe version of thread_create() to be used when the caller wants to
   * create a kernel thread to run within the current zone's context.
   */
  extern kthread_t *zthread_create(caddr_t, size_t, void (*)(), void *, size_t,
      pri_t);