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

*** 830,860 ****
   * and release them.
   */
  void
  dsl_pool_clean_tmp_userrefs(dsl_pool_t *dp)
  {
          zap_attribute_t za;
          zap_cursor_t zc;
          objset_t *mos = dp->dp_meta_objset;
          uint64_t zapobj = dp->dp_tmp_userrefs_obj;
  
          if (zapobj == 0)
                  return;
          ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);
  
          for (zap_cursor_init(&zc, mos, zapobj);
              zap_cursor_retrieve(&zc, &za) == 0;
              zap_cursor_advance(&zc)) {
-                 char *htag;
-                 uint64_t dsobj;
- 
                  htag = strchr(za.za_name, '-');
                  *htag = '\0';
                  ++htag;
                  dsobj = strtonum(za.za_name, NULL);
!                 dsl_dataset_user_release_tmp(dp, dsobj, htag);
          }
          zap_cursor_fini(&zc);
  }
  
  /*
   * Create the pool-wide zap object for storing temporary snapshot holds.
--- 830,879 ----
   * and release them.
   */
  void
  dsl_pool_clean_tmp_userrefs(dsl_pool_t *dp)
  {
+         char *htag;
          zap_attribute_t za;
          zap_cursor_t zc;
          objset_t *mos = dp->dp_meta_objset;
          uint64_t zapobj = dp->dp_tmp_userrefs_obj;
+         uint64_t dsobj;
+         nvlist_t *holds, *tags;
+         dsl_dataset_t *ds;
+         char name[MAXNAMELEN];
  
          if (zapobj == 0)
                  return;
          ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);
  
+         holds = fnvlist_alloc();
+ 
+         dsl_pool_config_enter(dp, FTAG);
          for (zap_cursor_init(&zc, mos, zapobj);
              zap_cursor_retrieve(&zc, &za) == 0;
              zap_cursor_advance(&zc)) {
                  htag = strchr(za.za_name, '-');
                  *htag = '\0';
                  ++htag;
                  dsobj = strtonum(za.za_name, NULL);
!                 if (dsl_dataset_hold_obj(dp, dsobj, FTAG, &ds) == 0) {
!                         dsl_dataset_name(ds, name);
!                         if (nvlist_lookup_nvlist(holds, name, &tags) != 0) {
!                                 tags = fnvlist_alloc();
!                                 fnvlist_add_boolean(tags, htag);
!                                 fnvlist_add_nvlist(holds, name, tags);
!                                 fnvlist_free(tags);
!                         } else {
!                                 fnvlist_add_boolean(tags, htag);
!                         }
!                         dsl_dataset_rele(ds, FTAG);
!                 }
          }
+         dsl_pool_config_exit(dp, FTAG);
+         dsl_dataset_user_release(holds, NULL);
+         fnvlist_free(holds);
          zap_cursor_fini(&zc);
  }
  
  /*
   * Create the pool-wide zap object for storing temporary snapshot holds.