md/dm-writecache: Don't request pointer dummy_addr when not required

Function persistent_memory_claim doesn't need to get local pointer
dummy_addr from direct_access. Using NULL instead of having to pass
in a useless local pointer that caller then just throw away.

Suggested-by: Ross Zwisler <[e-mail]>
Signed-off-by: Huaisheng Ye <[e-mail]>
Reviewed-by: Ross Zwisler <[e-mail]>
Acked-by: Mike Snitzer <[e-mail]>
Signed-off-by: Dave Jiang <[e-mail]>

drivers/md/dm-writecache.c

···
 		i = 0;
 		do {
 			long daa;
-			void *dummy_addr;
 			daa = dax_direct_access(wc->ssd_dev->dax_dev, i, p - i,
-						&dummy_addr, &pfn);
+						NULL, &pfn);
 			if (daa <= 0) {
 				r = daa ? daa : -EINVAL;
 				goto err3;