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

*** 41,52 ****
  
  int dsl_dataset_user_hold(nvlist_t *holds, minor_t cleanup_minor,
      nvlist_t *errlist);
  int dsl_dataset_user_release(nvlist_t *holds, nvlist_t *errlist);
  int dsl_dataset_get_holds(const char *dsname, nvlist_t *nvl);
- void dsl_dataset_user_release_tmp(struct dsl_pool *dp, uint64_t dsobj,
-     const char *htag);
  int dsl_dataset_user_hold_check_one(struct dsl_dataset *ds, const char *htag,
      boolean_t temphold, struct dmu_tx *tx);
  void dsl_dataset_user_hold_sync_one(struct dsl_dataset *ds, const char *htag,
      minor_t minor, uint64_t now, struct dmu_tx *tx);
  
--- 41,50 ----