OpenZFS 9421, 9422 - zdb show possibly leaked objects

9421 zdb should detect and print out the number of "leaked" objects
9422 zfs diff and zdb should explicitly mark objects that are on
     the deleted queue

It is possible for zfs to "leak" objects in such a way that they are not
freed, but are also not accessible via the POSIX interface. As the only
way to know that this is happened is to see one of them directly in a
zdb run, or by noting unaccounted space usage, zdb should be enhanced to
count these objects and return failure if some are detected.

We have access to the delete queue through the zfs_get_deleteq function;
we should call it in dump_znode to determine if the object is on the
delete queue. This is not the most efficient possible method, but it is
the simplest to implement, and should suffice for the common case where
there few objects on the delete queue.

Also zfs diff and zdb currently traverse every single dnode in a dataset
and tries to figure out the path of the object by following it's parent.
When an object is placed on the delete queue, for all practical purposes
it's already discarded, it's parent might not exist anymore, and another
object might now have the object number that belonged to the parent.
While all of the above makes sense, when trying to figure out the path
of an object that is on the delete queue, we can run into issues where
either it is impossible to determine the path because the parent is
gone, or another dnode has taken it's place and thus we are returned a
wrong path.

We should therefore avoid trying to determine the path of an object on
the delete queue and mark the object itself as being on the delete queue
to avoid confusion. To achieve this, we currently have two ideas:

1. When putting an object on the delete queue, change it's parent object
   number to a known constant that means NULL.

2. When displaying objects, first check if it is present on the delete
   queue.

Authored by: Paul Dagnelie <[email protected]>
Reviewed by: Matt Ahrens <[email protected]>
Reviewed by: Pavel Zakharov <[email protected]>
Approved by: Matt Ahrens <[email protected]>
Ported-by: Brian Behlendorf <[email protected]>

OpenZFS-issue: https://illumos.org/issues/9421
OpenZFS-issue: https://illumos.org/issues/9422
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/45ae0dd9ca
Closes #7500

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