blkcg: make ->cpd_init_fn() optional

mainline inclusion
from mainline-5.4-rc1
commit 86a5bba5c252e90d264c7460e29a0b9e633777e7
category: feature
bugzilla: 38688
CVE: NA

---------------------------

For policies which can do enough initialization from ->cpd_alloc_fn(),
make ->cpd_init_fn() optional.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NYu Kuai <yukuai3@huawei.com>
Reviewed-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

block/blk-cgroup.c

@@ -1570,6 +1570,7 @@ int blkcg_policy_register(struct blkcg_policy *pol)
 			blkcg->cpd[pol->plid] = cpd;
 			cpd->blkcg = blkcg;
 			cpd->plid = pol->plid;
+			if (pol->cpd_init_fn)
 				pol->cpd_init_fn(cpd);
 		}
 	}