null_blk: Fix: memory release when memory_backed=1

Memory/pages are not freed, when unloading nullblk driver.

Steps to reproduce issue
  1.free -h
        total        used        free      shared  buff/cache   available
Mem:    7.8Gi       260Mi       7.1Gi       3.0Mi       395Mi       7.3Gi
Swap:      0B          0B          0B
  2.modprobe null_blk memory_backed=1
  3.dd if=/dev/urandom of=/dev/nullb0 oflag=direct bs=1M count=1000
  4.modprobe -r null_blk
  5.free -h
        total        used        free      shared  buff/cache   available
Mem:    7.8Gi       1.2Gi       6.1Gi       3.0Mi       398Mi       6.3Gi
Swap:      0B          0B          0B

Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
Link: https://lore.kernel.org/r/20230605062354.24785-1-nj.shetty@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk/main.c

@@ -2244,6 +2244,7 @@ static void null_destroy_dev(struct nullb *nullb)
 	struct nullb_device *dev = nullb->dev;
 
 	null_del_dev(nullb);
+	null_free_device_storage(dev, false);
 	null_free_dev(dev);
 }