md/md-cluster: cleanup md_cluster_ops reference

md_cluster_ops->slot_number() is implemented inside md-cluster.c, just
call it directly.

Link: https://lore.kernel.org/linux-raid/20250215092225.2427977-6-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Su Yue <glass.su@suse.com>