librbd: don't clip expanded diff on truncate in ObjectListSnapsRequest

librbd: don't clip expanded diff on truncate in ObjectListSnapsRequest

If the diff was expanded due to LIST_SNAPS_FLAG_WHOLE_OBJECT, clipping
it when handling a truncate is wrong -- when subtracting that interval,
we either split the expanded extent into two or chop off a piece of it.
However the point of LIST_SNAPS_FLAG_WHOLE_OBJECT is to report a single
extent covering the entire object.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 0ebca5fe22d976f1a17f8078f20a47c49a057edb)