rbd: remove VLA usage

As part of the effort to remove VLAs from the kernel[1], this moves
the literal values into the stack array calculation instead of using a
variable for the sizing. The resulting size can be found from
sizeof(buf).

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Kyle Spiers <kyle@spiers.me>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -2683,8 +2683,8 @@ static int __rbd_notify_op_lock(struct rbd_device *rbd_dev,
 {
 	struct ceph_osd_client *osdc = &rbd_dev->rbd_client->client->osdc;
 	struct rbd_client_id cid = rbd_get_cid(rbd_dev);
-	int buf_size = 4 + 8 + 8 + CEPH_ENCODING_START_BLK_LEN;
-	char buf[buf_size];
+	char buf[4 + 8 + 8 + CEPH_ENCODING_START_BLK_LEN];
+	int buf_size = sizeof(buf);
 	void *p = buf;
 
 	dout("%s rbd_dev %p notify_op %d\n", __func__, rbd_dev, notify_op);
@@ -3202,8 +3202,8 @@ static void __rbd_acknowledge_notify(struct rbd_device *rbd_dev,
 				     u64 notify_id, u64 cookie, s32 *result)
 {
 	struct ceph_osd_client *osdc = &rbd_dev->rbd_client->client->osdc;
-	int buf_size = 4 + CEPH_ENCODING_START_BLK_LEN;
-	char buf[buf_size];
+	char buf[4 + CEPH_ENCODING_START_BLK_LEN];
+	int buf_size = sizeof(buf);
 	int ret;
 
 	if (result) {