dm vdo flush: initialize return to NULL in allocate_flush

Otherwise, error path could result in allocate_flush's subsequent
check for flush being non-NULL leading to false positive.
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Susan LeGendre-McGhee <slegendr@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-vdo/flush.c

@@ -100,7 +100,7 @@ static struct vdo_flush *vdo_waiter_as_flush(struct vdo_waiter *waiter)
 
 static void *allocate_flush(gfp_t gfp_mask, void *pool_data)
 {
-	struct vdo_flush *flush;
+	struct vdo_flush *flush = NULL;
 
 	if ((gfp_mask & GFP_NOWAIT) == GFP_NOWAIT) {
 		flush = uds_allocate_memory_nowait(sizeof(struct vdo_flush), __func__);