dm writecache: correct uncommitted_block when discarding uncommitted entry

stable inclusion
from linux-4.19.131
commit a33e2d0aa4e265ae5ea06778516b863b17a7c737

--------------------------------

commit 39495b12 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>