bcache: avoid nr_stripes overflow in bcache_device_init()

For some block devices which large capacity (e.g. 8TB) but small io_opt
size (e.g. 8 sectors), in bcache_device_init() the stripes number calcu-
lated by,
	DIV_ROUND_UP_ULL(sectors, d->stripe_size);
might be overflow to the unsigned int bcache_device->nr_stripes.

This patch uses the uint64_t variable to store DIV_ROUND_UP_ULL()
and after the value is checked to be available in unsigned int range,
sets it to bache_device->nr_stripes. Then the overflow is avoided.
Reported-and-tested-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Coly Li <colyli@suse.de>
Cc: stable@vger.kernel.org
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1783075Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -835,19 +835,19 @@ static int bcache_device_init(struct bcache_device *d, unsigned int block_size,
 	struct request_queue *q;
 	const size_t max_stripes = min_t(size_t, INT_MAX,
 					 SIZE_MAX / sizeof(atomic_t));
-	size_t n;
+	uint64_t n;
 	int idx;
 
 	if (!d->stripe_size)
 		d->stripe_size = 1 << 31;
 
-	d->nr_stripes = DIV_ROUND_UP_ULL(sectors, d->stripe_size);
-
-	if (!d->nr_stripes || d->nr_stripes > max_stripes) {
-		pr_err("nr_stripes too large or invalid: %u (start sector beyond end of disk?)\n",
-			(unsigned int)d->nr_stripes);
+	n = DIV_ROUND_UP_ULL(sectors, d->stripe_size);
+	if (!n || n > max_stripes) {
+		pr_err("nr_stripes too large or invalid: %llu (start sector beyond end of disk?)\n",
+			n);
 		return -ENOMEM;
 	}
+	d->nr_stripes = n;
 
 	n = d->nr_stripes * sizeof(atomic_t);
 	d->stripe_sectors_dirty = kvzalloc(n, GFP_KERNEL);