nbd: fix 64-bit division

We have this:

ERROR: "__aeabi_ldivmod" [drivers/block/nbd.ko] undefined!
ERROR: "__divdi3" [drivers/block/nbd.ko] undefined!
nbd.c:(.text+0x247c72): undefined reference to `__divdi3'

due to a recent commit, that did 64-bit division. Use the proper
divider function so that 32-bit compiles don't break.

Fixes: ef77b51 ("nbd: use loff_t for blocksize and nbd_set_size args")
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nbd.c

@@ -729,7 +729,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
 
 	case NBD_SET_SIZE:
 		return nbd_size_set(nbd, bdev, nbd->blksize,
-				    arg / nbd->blksize);
+					div_s64(arg, nbd->blksize));
 
 	case NBD_SET_SIZE_BLOCKS:
 		return nbd_size_set(nbd, bdev, nbd->blksize, arg);