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

*** 593,602 ****
--- 593,605 ----
      sendflags_t *, int, snapfilter_cb_t, void *, nvlist_t **);
  
  extern int zfs_promote(zfs_handle_t *);
  extern int zfs_hold(zfs_handle_t *, const char *, const char *,
      boolean_t, boolean_t, int);
+ extern int zfs_hold_add(zfs_handle_t *, const char *, const char *,
+     boolean_t, nvlist_t *);
+ extern int zfs_hold_apply(zfs_handle_t *, boolean_t, int, nvlist_t *);
  extern int zfs_release(zfs_handle_t *, const char *, const char *, boolean_t);
  extern int zfs_get_holds(zfs_handle_t *, nvlist_t **);
  extern uint64_t zvol_volsize_to_reservation(uint64_t, nvlist_t *);
  
  typedef int (*zfs_userspace_cb_t)(void *arg, const char *domain,