5047 don't use atomic_*_nv if you discard the return value

*** 282,292 ****
          db = dn->dn_bonus;
  
          /* as long as the bonus buf is held, the dnode will be held */
          if (refcount_add(&db->db_holds, tag) == 1) {
                  VERIFY(dnode_add_ref(dn, db));
!                 (void) atomic_inc_32_nv(&dn->dn_dbufs_count);
          }
  
          /*
           * Wait to drop dn_struct_rwlock until after adding the bonus dbuf's
           * hold and incrementing the dbuf count to ensure that dnode_move() sees
--- 282,292 ----
          db = dn->dn_bonus;
  
          /* as long as the bonus buf is held, the dnode will be held */
          if (refcount_add(&db->db_holds, tag) == 1) {
                  VERIFY(dnode_add_ref(dn, db));
!                 atomic_inc_32(&dn->dn_dbufs_count);
          }
  
          /*
           * Wait to drop dn_struct_rwlock until after adding the bonus dbuf's
           * hold and incrementing the dbuf count to ensure that dnode_move() sees