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

@@ -20,11 +20,11 @@
  */
 
 /*
  * 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 Joyent, Inc.
  * Copyright 2019 Nexenta Systems, Inc. All rights reserved.
  */
 
 #ifndef _SYS_ZONE_H
 #define _SYS_ZONE_H

@@ -780,14 +780,29 @@
  */
 #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()   (curproc->p_zone->zone_rootvp)
+#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);