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.