3006 VERIFY[S,U,P] and ASSERT[S,U,P] frequently check if first argument is zero

*** 767,777 ****
          }
  
          for (int t = 0; t < TXG_DEFER_SIZE; t++)
                  space_map_destroy(&msp->ms_defermap[t]);
  
!         ASSERT3S(msp->ms_deferspace, ==, 0);
  
          mutex_exit(&msp->ms_lock);
          mutex_destroy(&msp->ms_lock);
  
          kmem_free(msp, sizeof (metaslab_t));
--- 767,777 ----
          }
  
          for (int t = 0; t < TXG_DEFER_SIZE; t++)
                  space_map_destroy(&msp->ms_defermap[t]);
  
!         ASSERT0(msp->ms_deferspace);
  
          mutex_exit(&msp->ms_lock);
          mutex_destroy(&msp->ms_lock);
  
          kmem_free(msp, sizeof (metaslab_t));