librbd: make writesame get all zeros with rbd_skip_partial_discard on

librbd: make writesame get all zeros with rbd_skip_partial_discard on

Now writesame tends to do a discard on all zero buffer, but
if rbd_skip_partial_discard is turned on, discard will not do any
zeroing work on partial discard, so we read random data while expecting
all zeros.
To fix it, we introduce the ImageCtx::skip_partial_discard and
pass an extra parameter to discard. Writesame will always pass false
and other discards will pass the ImageCtx::skip_partial_discard
which is set by the rbd_skip_partial_discard option.

Signed-off-by: Gui Hecheng <guihecheng@cmss.chinamobile.com>