blkcg: add missing NULL check in ioc_cpd_alloc()

ioc_cpd_alloc() forgot to check NULL return from kzalloc().  Add it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -1888,8 +1888,10 @@ static struct blkcg_policy_data *ioc_cpd_alloc(gfp_t gfp)
 	struct ioc_cgrp *iocc;
 
 	iocc = kzalloc(sizeof(struct ioc_cgrp), gfp);
-	iocc->dfl_weight = CGROUP_WEIGHT_DFL;
+	if (!iocc)
+		return NULL;
 
+	iocc->dfl_weight = CGROUP_WEIGHT_DFL;
 	return &iocc->cpd;
 }