librbd: propagate diff-iterate range to parent in fast-diff mode

librbd: propagate diff-iterate range to parent in fast-diff mode

When getting parent diff, pass the overlap-reduced image extent instead
of the entire 0..overlap range to avoid a similar quadratic slowdown on
cloned images.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 7677d4b1b52ab68484545d0fcd7348f2f8e9f263)

Conflicts:
	src/librbd/api/DiffIterate.cc [ ImageArea support not in
	  pacific ]