5056 ZFS deadlock on db_mtx and dn_holds
Reviewed by: Will Andrews <willa@spectralogic.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

*** 350,360 ****
          uint64_t zapobj;
          nvlist_t *holds_found;
          objset_t *mos;
          int numholds;
  
!         if (!dsl_dataset_is_snapshot(ds))
                  return (SET_ERROR(EINVAL));
  
          if (nvlist_empty(holds))
                  return (0);
  
--- 350,360 ----
          uint64_t zapobj;
          nvlist_t *holds_found;
          objset_t *mos;
          int numholds;
  
!         if (!ds->ds_is_snapshot)
                  return (SET_ERROR(EINVAL));
  
          if (nvlist_empty(holds))
                  return (0);