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_nvl instead of processing each snapshot in turn.

Deferred (on exit) cleanup by the kernel is also now done in dsl_sync_task by
reusing dsl_dataset_user_release_tmp.

lzc_hold and dsl_dataset_user_hold now skip failed holds for non-existent
snapshots, however they ensure at least one of the holds succeeded.

See the comment above lzc_hold() for full description on the new behaviour.

Eliminate enoent_ok which is no longer needed with the new process flow.

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