librbd: diff-iterate shouldn't ever report "new hole" against a hole

librbd: diff-iterate shouldn't ever report "new hole" against a hole

If an object doesn't exist in both start and end versions but there is
an intermediate snapshot which contains it (i.e. the object is written
to and captured at some point but then discarded prior to or in the end
version), diff-iterate reports "new hole" -- callback is invoked with
exists=false.  This occurs both on the slow list_snaps path and in
fast-diff mode.

Despite going all the way back to the introduction of diff-iterate in
commit 0296c7cdae91 ("librbd: implement diff_iterate"), this behavior
is wrong and contradicts diff-iterate API documentation added in commit
a69532e86450 ("librbd: document diff_iterate in header") in the same
series:

    If the source snapshot name is NULL, we interpret that as
    the beginning of time and return all allocated regions of the
    image.

It also triggered an assert added in commit c680531e070a ("librbd:
change diff_iterate interface to be more C-friendly") in the same
series.  Unfortunately, commit f1f6407221a0 ("test_librbd: add
diff_iterate test including discard"), also part of the same series,
added a test which expected the wrong behavior.  Very confusing!

A year later, a different manifestation of this bug was fixed in commit
9a1ab95176fe ("rbd: Fix rbd diff for non-existent objects"), but the
fix only covered the case where calc_snap_set_diff() goes past the end
snap ID while processing clones.  The case where it runs out of clones
to process before reaching the end snap ID remained mishandled.

A year after that, commit 3ccc3bb4bd35 ("librbd: diff_iterate needs to
handle holes in parent images") dropped the assert mentioned above and
this bug got enshrined in the newly introduced fast-diff mode.

Finally, a few years later, deep-copy actually started relying on this
bug in commit e5a21e904142 ("librbd: deep-copy image copy state machine
skips clean objects").  This necessitates bifurcation in DiffRequest
because deep-copy wants the "has this object been touched" semantics,
which is different from diff-iterate (and also potentially much more
expensive to produce!).

This commit brings a minimal update to TestMockObjectMapDiffRequest
tests and DiffIterateTest.DiffIterateDiscard.  Coverage is expanded in
the following commits.

Fixes: https://tracker.ceph.com/issues/53897
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>