dm cache: handle kmalloc failure allocating background_tracker struct

Currently there is no kmalloc failure check on the allocation of
the background_tracker struct in btracker_create(), and so a NULL return
will lead to a NULL pointer dereference.  Add a NULL check.

Detected by CoverityScan, CID#1416587 ("Dereference null return value")

Fixes: b29d4986d ("dm cache: significant rework to leverage dm-bio-prison-v2")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-background-tracker.c

@@ -33,6 +33,11 @@ struct background_tracker *btracker_create(unsigned max_work)
 {
 	struct background_tracker *b = kmalloc(sizeof(*b), GFP_KERNEL);
 
+	if (!b) {
+		DMERR("couldn't create background_tracker");
+		return NULL;
+	}
+
 	b->max_work = max_work;
 	atomic_set(&b->pending_promotes, 0);
 	atomic_set(&b->pending_writebacks, 0);