Optimize creation and removal of temporary "user holds" placed on
snapshots by a zfs send, by ensuring all the required holds and
releases are done in a single dsl_sync_task.
Creation now collates the required holds during a dry run and
then uses a single lzc_hold call via zfs_hold_apply instead of
processing each snapshot in turn.
Defered (on exit) cleanup by the kernel is also now done in
dsl_sync_task by reusing dsl_dataset_user_release.
On a test with 11 volumes in a tree each with 8 snapshots on a
single HDD zpool this reduces the time required to perform a full
send from 20 seconds to under 0.8 seconds.
For reference eliminating the hold entirely reduces this 0.15
seconds.
While I'm here:-
* Remove some unused structures
* Fix nvlist_t leak in zfs_release_one

*** 185,196 ****
  int dsl_dataset_own_obj(struct dsl_pool *dp, uint64_t dsobj,
      void *tag, dsl_dataset_t **dsp);
  void dsl_dataset_disown(dsl_dataset_t *ds, void *tag);
  void dsl_dataset_name(dsl_dataset_t *ds, char *name);
  boolean_t dsl_dataset_tryown(dsl_dataset_t *ds, void *tag);
! void dsl_register_onexit_hold_cleanup(dsl_dataset_t *ds, const char *htag,
!     minor_t minor);
  uint64_t dsl_dataset_create_sync(dsl_dir_t *pds, const char *lastname,
      dsl_dataset_t *origin, uint64_t flags, cred_t *, dmu_tx_t *);
  uint64_t dsl_dataset_create_sync_dd(dsl_dir_t *dd, dsl_dataset_t *origin,
      uint64_t flags, dmu_tx_t *tx);
  int dsl_dataset_snapshot(nvlist_t *snaps, nvlist_t *props, nvlist_t *errors);
--- 185,195 ----
  int dsl_dataset_own_obj(struct dsl_pool *dp, uint64_t dsobj,
      void *tag, dsl_dataset_t **dsp);
  void dsl_dataset_disown(dsl_dataset_t *ds, void *tag);
  void dsl_dataset_name(dsl_dataset_t *ds, char *name);
  boolean_t dsl_dataset_tryown(dsl_dataset_t *ds, void *tag);
! void dsl_register_onexit_hold_cleanup(nvlist_t *holds, minor_t minor);
  uint64_t dsl_dataset_create_sync(dsl_dir_t *pds, const char *lastname,
      dsl_dataset_t *origin, uint64_t flags, cred_t *, dmu_tx_t *);
  uint64_t dsl_dataset_create_sync_dd(dsl_dir_t *dd, dsl_dataset_t *origin,
      uint64_t flags, dmu_tx_t *tx);
  int dsl_dataset_snapshot(nvlist_t *snaps, nvlist_t *props, nvlist_t *errors);