dm-writecache: fix uncommitted_blocks underflow

Revert the commit 39495b12ef

writecache_bio_copy_ssd sets wc->uncommitted_blocks = 0; and then queues
the flush work: queue_work(wc->writeback_wq, &wc->flush_work);

It may be possible that discard is dispatched in this window, and the
discard would decrement wc->uncommitted_blocks to -1U.

Note that this bug is benign - wc->uncommitted_blocks is only used to
trigged autocommit, so decrementing it to -1U will trigger a spurious
autocommit, but won't cause any data corruption.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-5
Fixes: 39495b12ef ("dm writecache: correct uncommitted_block when discarding uncommitted entry")
Cc: stable@vger.kernel.org

drivers/md/dm-writecache.c

@@ -901,8 +901,6 @@ static void writecache_discard(struct dm_writecache *wc, sector_t start, sector_
 				}
 				discarded_something = true;
 			}
-			if (!writecache_entry_is_committed(wc, e))
-				wc->uncommitted_blocks--;
 			writecache_free_entry(wc, e);
 		}