dm writecache: remove superfluous test in persistent_memory_claim

Remove superfluous test if dax_dev is NULL - dax_direct_access already
does this test.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -234,10 +234,6 @@ static int persistent_memory_claim(struct dm_writecache *wc)
 
 	wc->memory_vmapped = false;
 
-	if (!wc->ssd_dev->dax_dev) {
-		r = -EOPNOTSUPP;
-		goto err1;
-	}
 	s = wc->memory_map_size;
 	p = s >> PAGE_SHIFT;
 	if (!p) {