dm writecache: correct uncommitted_block when discarding uncommitted entry

When uncommitted entry has been discarded, correct wc->uncommitted_block
for getting the exact number.

Fixes: 48debafe ("dm: add writecache target")
Cc: stable@vger.kernel.org
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>