block, bfq: return nbytes and not zero from struct cftype .write() method

commit fc8ebd01 upstream.

The value that struct cftype .write() method returns is then directly
returned to userspace as the value returned by write() syscall, so it
should be the number of bytes actually written (or consumed) and not zero.

Returning zero from write() syscall makes programs like /bin/echo or bash
spin.
Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Fixes: e21b7a0b ("block, bfq: add full hierarchical scheduling and cgroups support")
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

block/bfq-cgroup.c

@@ -887,7 +887,8 @@ static ssize_t bfq_io_set_weight(struct kernfs_open_file *of,
 	if (ret)
 		return ret;
 
-	return bfq_io_set_weight_legacy(of_css(of), NULL, weight);
+	ret = bfq_io_set_weight_legacy(of_css(of), NULL, weight);
+	return ret ?: nbytes;
 }
 
 static int bfqg_print_stat(struct seq_file *sf, void *v)