cfq-iosched: fix sysfs oops when attempting to read unconfigured weights

If none of the devices in the system are using CFQ, then attempting to
read:

/sys/fs/cgroup/blkio/blkio.leaf_weight

will results in a NULL dereference. Check for a valid cfq_group_data
struct before attempting to dereference it.

Reported-by: Andrey Wagin <avagin@gmail.com>
Fixes: e48453c3 ("block, cgroup: implement policy-specific per-blkcg data")
Signed-off-by: Jens Axboe <axboe@fb.com>

block/cfq-iosched.c

@@ -1714,16 +1714,26 @@ static int cfqg_print_leaf_weight_device(struct seq_file *sf, void *v)
 static int cfq_print_weight(struct seq_file *sf, void *v)
 {
 	struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
+	struct cfq_group_data *cgd = blkcg_to_cfqgd(blkcg);
+	unsigned int val = 0;
 
-	seq_printf(sf, "%u\n", blkcg_to_cfqgd(blkcg)->weight);
+	if (cgd)
+		val = cgd->weight;
+
+	seq_printf(sf, "%u\n", val);
 	return 0;
 }
 
 static int cfq_print_leaf_weight(struct seq_file *sf, void *v)
 {
 	struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
+	struct cfq_group_data *cgd = blkcg_to_cfqgd(blkcg);
+	unsigned int val = 0;
 
-	seq_printf(sf, "%u\n", blkcg_to_cfqgd(blkcg)->leaf_weight);
+	if (cgd)
+		val = cgd->leaf_weight;
+
+	seq_printf(sf, "%u\n", val);
 	return 0;
 }