Revert exi_zone to exi_zoneid, and install exi_ne backpointer

*** 876,886 ****
  
          avl_index_t             where;  /* used for avl_find()/avl_insert() */
  
          ASSERT(cr != NULL);
  
!         ASSERT3P(curzone, ==, exi->exi_zone);
          nag = nfsauth_get_zg();
  
          /*
           * Now check whether this client already
           * has an entry for this flavor in the cache
--- 876,886 ----
  
          avl_index_t             where;  /* used for avl_find()/avl_insert() */
  
          ASSERT(cr != NULL);
  
!         ASSERT3P(curzone->zone_id, ==, exi->exi_zoneid);
          nag = nfsauth_get_zg();
  
          /*
           * Now check whether this client already
           * has an entry for this flavor in the cache