librados/snap_set_diff: set end_size only if end object exists

librados/snap_set_diff: set end_size only if end object exists

Since commit 73f50a13109f ("rbd-mirror: use generalized deep copy for
image sync"), the only user of calc_snap_set_diff() immediately unsets
end_size otherwise.

calc_snap_set_diff() semantics are clearer if end_size is set together
with end_exists and clone_end_snap_id.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit c0747922040841c371dbe2706354c08c73d8e59f)