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>

*** 49,59 ****
  } dmu_snapshots_destroy_arg_t;
  
  int
  dsl_destroy_snapshot_check_impl(dsl_dataset_t *ds, boolean_t defer)
  {
!         if (!dsl_dataset_is_snapshot(ds))
                  return (SET_ERROR(EINVAL));
  
          if (dsl_dataset_long_held(ds))
                  return (SET_ERROR(EBUSY));
  
--- 49,59 ----
  } dmu_snapshots_destroy_arg_t;
  
  int
  dsl_destroy_snapshot_check_impl(dsl_dataset_t *ds, boolean_t defer)
  {
!         if (!ds->ds_is_snapshot)
                  return (SET_ERROR(EINVAL));
  
          if (dsl_dataset_long_held(ds))
                  return (SET_ERROR(EBUSY));
  
*** 352,362 ****
  
          /* Collapse range in clone heads */
          dsl_dataset_remove_clones_key(ds,
              dsl_dataset_phys(ds)->ds_creation_txg, tx);
  
!         if (dsl_dataset_is_snapshot(ds_next)) {
                  dsl_dataset_t *ds_nextnext;
  
                  /*
                   * Update next's unique to include blocks which
                   * were previously shared by only this snapshot
--- 352,362 ----
  
          /* Collapse range in clone heads */
          dsl_dataset_remove_clones_key(ds,
              dsl_dataset_phys(ds)->ds_creation_txg, tx);
  
!         if (ds_next->ds_is_snapshot) {
                  dsl_dataset_t *ds_nextnext;
  
                  /*
                   * Update next's unique to include blocks which
                   * were previously shared by only this snapshot
*** 602,613 ****
  {
          int error;
          uint64_t count;
          objset_t *mos;
  
!         ASSERT(!dsl_dataset_is_snapshot(ds));
!         if (dsl_dataset_is_snapshot(ds))
                  return (SET_ERROR(EINVAL));
  
          if (refcount_count(&ds->ds_longholds) != expected_holds)
                  return (SET_ERROR(EBUSY));
  
--- 602,613 ----
  {
          int error;
          uint64_t count;
          objset_t *mos;
  
!         ASSERT(!ds->ds_is_snapshot);
!         if (ds->ds_is_snapshot)
                  return (SET_ERROR(EINVAL));
  
          if (refcount_count(&ds->ds_longholds) != expected_holds)
                  return (SET_ERROR(EBUSY));