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>

*** 472,482 ****
  zilog_dirty(zilog_t *zilog, uint64_t txg)
  {
          dsl_pool_t *dp = zilog->zl_dmu_pool;
          dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);
  
!         if (dsl_dataset_is_snapshot(ds))
                  panic("dirtying snapshot!");
  
          if (txg_list_add(&dp->dp_dirty_zilogs, zilog, txg)) {
                  /* up the hold count until we can be written out */
                  dmu_buf_add_ref(ds->ds_dbuf, zilog);
--- 472,482 ----
  zilog_dirty(zilog_t *zilog, uint64_t txg)
  {
          dsl_pool_t *dp = zilog->zl_dmu_pool;
          dsl_dataset_t *ds = dmu_objset_ds(zilog->zl_os);
  
!         if (ds->ds_is_snapshot)
                  panic("dirtying snapshot!");
  
          if (txg_list_add(&dp->dp_dirty_zilogs, zilog, txg)) {
                  /* up the hold count until we can be written out */
                  dmu_buf_add_ref(ds->ds_dbuf, zilog);