librbd: restore diff-iterate include_parent functionality in fast-diff mode

librbd: restore diff-iterate include_parent functionality in fast-diff mode

Commit 4429ed4f3f4c ("librbd: switch diff iterate API to use new snaps
list dispatch methods") removed the recursive execute() call.  The new
list_snaps method does indeed handle parent diffs internally but it is
not used in fast-diff mode.  Nothing changed there -- we still need to
load the parent object map, calculate parent object_diff_state, etc.

Fixes: https://tracker.ceph.com/issues/53787
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 04293bef6ccd2b9ca3db53906b63c952e235cdb4)

Conflicts:
	src/librbd/api/DiffIterate.cc [ drop all changes, bring in
	  just the new test ]