dm writecache: fix unnecessary NULL check warnings

Remove NULL checks before vfree() to fix these warnings:
./drivers/md/dm-writecache.c:2008:2-7: WARNING: NULL check before some
freeing functions is not needed.
./drivers/md/dm-writecache.c:2024:2-7: WARNING: NULL check before some
freeing functions is not needed.
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -2004,7 +2004,6 @@ static void writecache_dtr(struct dm_target *ti)
 	if (wc->ssd_dev)
 		dm_put_device(ti, wc->ssd_dev);
 
-	if (wc->entries)
 	vfree(wc->entries);
 
 	if (wc->memory_map) {
@@ -2020,7 +2019,6 @@ static void writecache_dtr(struct dm_target *ti)
 	if (wc->dm_io)
 		dm_io_client_destroy(wc->dm_io);
 
-	if (wc->dirty_bitmap)
 	vfree(wc->dirty_bitmap);
 
 	kfree(wc);