nbd: use correct div_s64 helper

The do_div() macro now checks its arguments for the correct type,
and refuses anything other than u64, so we get a warning about
nbd_ioctl passing in an loff_t:

drivers/block/nbd.c: In function '__nbd_ioctl':
drivers/block/nbd.c:757:77: error: comparison of distinct pointer types lacks a cast [-Werror]

This changes the nbd code to use div_s64() instead, which takes
a signed argument.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Fixes: 37091fdd ("nbd: Create size change events for userspace")
Signed-off-by: NJens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -753,8 +753,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 	}
 
 	case NBD_SET_BLKSIZE: {
-		loff_t bsize = nbd->bytesize;
-		do_div(bsize, arg);
+		loff_t bsize = div_s64(nbd->bytesize, arg);
 
 		return nbd_size_set(nbd, bdev, arg, bsize);
 	}